All of Isaiah 11:6-9 has already been fulfilled, all we have to do is read it in context with the rest of the chapter. Many early church fathers interpreted it as being fulfilled at the first coming of Christ. For example, I own the commentary on Isaiah by Eusebius of Caesarea from the "Ancient Christian Texts" series, where he interprets it as already fulfilled. But there is one with way more authority than Eusebius that interprets it as fulfilled already, and that's the Apostle Paul. I will prove these things in my answer.
Let's first put Isaiah 11:8 in some context:
Isaiah 11 King James Version (KJV)
1 And there shall come forth a rod out of the stem of Jesse, and a
Branch shall grow out of his roots:
2 And the spirit of the Lord shall rest upon him, the spirit of wisdom
and understanding, the spirit of counsel and might, the spirit of
knowledge and of the fear of the Lord;
3 And shall make him of quick understanding in the fear of the Lord:
and he shall not judge after the sight of his eyes, neither reprove
after the hearing of his ears:
4 But with righteousness shall he judge the poor, and reprove with
equity for the meek of the earth: and he shall smite the earth: with
the rod of his mouth, and with the breath of his lips shall he slay
the wicked.
5 And righteousness shall be the girdle of his loins, and faithfulness
the girdle of his reins.
6 The wolf also shall dwell with the lamb, and the leopard shall lie
down with the kid; and the calf and the young lion and the fatling
together; and a little child shall lead them.
7 And the cow and the bear shall feed; their young ones shall lie down
together: and the lion shall eat straw like the ox.
8 And the sucking child shall play on the hole of the asp, and the
weaned child shall put his hand on the cockatrice' den.
9 They shall not hurt nor destroy in all my holy mountain: for the
earth shall be full of the knowledge of the Lord, as the waters cover
the sea.
10 And in that day there shall be a root of Jesse, which shall stand
for an ensign of the people; to it shall the Gentiles seek: and his
rest shall be glorious.
11 And it shall come to pass in that day, that the Lord shall set his
hand again the second time to recover the remnant of his people, which
shall be left, from Assyria, and from Egypt, and from Pathros, and
from Cush, and from Elam, and from Shinar, and from Hamath, and from
the islands of the sea.
12 And he shall set up an ensign for the nations, and shall assemble
the outcasts of Israel, and gather together the dispersed of Judah
from the four corners of the earth.
13 The envy also of Ephraim shall depart, and the adversaries of Judah
shall be cut off: Ephraim shall not envy Judah, and Judah shall not
vex Ephraim
In verses 1-5 We can see this chapter begins with the incarnation of the Messiah that would come from the bloodline of Jesse ( king David's father). It shows that This Messiah ( anointed one) would be anointed with the Holy Spirit, the Spirit resting on him was fulfilled at His baptism when the Holy Spirit descended on Yahuwshuwa ( Jesus) in the shape of a dove ( Mark 1:11).Then it says that he would judge righteously, not after his own sight or hearing. This is showing he would Judge according to His Father's will, not his own.
Then verse 6 shows:
1. the wolf shall dwell with the lamb
2. the leopard shall lie down with the kid
3. the calf and the young lion and the fatling together, a little child shall lead them.
first thing to notice is that in each of these, its an unclean animal together with a clean animal. the second thing is the peace between these animals, when normally they wouldn't be. and third, a little child will lead them.
In Acts 10:9-16 Peter had a vision showing unclean animals descending from heaven, which he told to kill and eat. In Acts 10:28 Peter shows us the interpretation of this when he says "
And he said to them, You know how that it is an unlawful thing for a
man that is a Jew to keep company, or come to one of another nation;
but God has showed me that I should not call any man common or
unclean.
So these unclean animals in the vision represent the gentiles, who were considered clean now. In isaiah 11 these wild unclean animals also represent gentiles, who now are keeping company with the clean animal (Jews) together peacefully in the church. These wild savage flesh eating beast are now peaceful, which fulfills Isaiah's other prophecy concerning the gentiles in chapter two.
Isaiah chapter 2:2-4
And it shall come to pass in the last days, that the mountain of the
Lord's house shall be established in the top of the mountains, and
shall be exalted above the hills; and all nations shall flow unto it.
And many people shall go and say, Come ye, and let us go up to the
mountain of the Lord, to the house of the God of Jacob; and he will
teach us of his ways, and we will walk in his paths: for out of Zion
shall go forth the law, and the word of the Lord from Jerusalem. And
he shall judge among the nations, and shall rebuke many people: and
they shall beat their swords into plowshares, and their spears into
pruninghooks: nation shall not lift up sword against nation, neither
shall they learn war any more
So these lions, leopards, and wolves putting away their violent natures are these same gentiles who beat their swords into plowshares and learned war no more after the hearing of the gospel.
Then it says a little child would lead them. This is also symbolic. Yahuwshuwa said in Mathew 18:3-4:
Truly I tell you,” He said, “unless you change and become like little
children, you will never enter the kingdom of heaven.Therefore,
whoever humbles himself like this little child is the greatest in the
kingdom of heaven.
Then Paul says this in 1 Corinthians 14:20
"Brethen , be not children in understanding: howbeit in malice be ye
children, but in understanding be men".
So the little child in Isaiah 11 represents those leaders in the church who made themselves as little children, who are infants in regard to evil.
Quoting from Esubuis' commentary on Isaiah 11:6 he says concerning the little child leading the beasts " And we are to understand that those in the church of Christ who possess deeper insight than politicians because of their simplicity of soul and guilelessness in reality are no different from infants, Nevertheless, these infants govern those gathered together from ever race"
Now verse 7 says that these animals would all eat together peacefully, not flesh but straw. This is symbolically showing them in fellowship nourishing their souls together from the divine scriptures. Again showing unclean and clean animals together.
Now we finally reached the verse in question. in verse 8 it says "And the sucking child shall play on the hole of the asp, and the weaned child shall put his hand on the cockatrice' den". now that we interpreted what the infant child represents, we can focus on what it means that he is not hurt puttng his hands among the serpents. lets look at a few verses:
Luke 10:19 Look, I have given you the authority to trample on snakes
and scorpions and over all the power of the enemy; nothing will ever
harm you.
Mark 16:17-18 " And these signs shall follow them that believe; In my
name shall they cast out devils; they shall speak with new tongues;
They shall take up serpents; and if they drink any deadly thing, it
shall not hurt them; they shall lay hands on the sick, and they shall
recover
Psalm 91:13 "Thou shalt tread upon the lion and adder: the young lion
and the dragon shalt thou trample under feet.
So these young children of Christ have been giving authority to tread over serpents, which are the demons, and nothing by any means would harm them. This is a reality that is fulfilled today.
verse 9 says nothing shall destroy in Yahuwahs Holy mountain, this is the same mountain i quoted from Isaiah 2. And it says the earth would be full of the knowlege of Yahuwah, this was fulfilled by the preaching of the apostles going out to the gentiles fulfilling psalms 19:2-4 "Day unto day uttereth speech, and night unto night sheweth knowledge.
There is no speech nor language, where their voice is not heard.
Their line is gone out through all the earth, and their words to the end of the world. In them hath he set a tabernacle for the sun".
Now verse 9 is where we can prove that these verses have already been fulfilled, it says " And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious". So it is saying IN THAT DAY, the day of the preceding verses of the animals, the gentiles shall seek the root of Jesse. The Apostle Paul declares these verses already fulfilled in Romans 15:8-12
Now I say that Jesus Christ was a minister of the circumcision for
the truth of God, to confirm the promises made unto the fathers:
9 And that the Gentiles might glorify God for his mercy; as it is
written, For this cause I will confess to thee among the Gentiles, and
sing unto thy name.
10 And again he saith, Rejoice, ye Gentiles, with his people.
11 And again, Praise the Lord, all ye Gentiles; and laud him, all ye
people.
12 And again, Esaias saith, There shall be a root of Jesse, and he
that shall rise to reign over the Gentiles; in him shall the Gentiles
trust.
So according to Paul, which he quoted Isaiah 11:1 and 11:10, it has already been fulfilled. and if Isaiah 11:10 is fulfilled, then that means Isaiah 11:8 is also fulfilled. Because Isaiah said in that day that the animals would come together, the gentiles would seek the root of Jesse.
then verses 11-13 talks about the recovering of the remnant of his people from among the nations through the gospel, as shown in John 11:51-52:
51 And this spake he not of himself: but being high priest that year,
he prophesied that Jesus should die for that nation;
52 And not for that nation only, but that also he should gather
together in one the children of God that were scattered abroad.
Then it talks the divided northern and southern kingdom of the Ephraim and Judah would be united, which connects to Ezekiel 37:16-17. This is talking about them coming together in peace through the gospel in the church, for all are one in Christ (Ephesians 2:13, Galatians 3:28).
so we can see that Isaiah 11:8 and its surrounding verses have all been fulfilled at the first coming to Yahuwshuwa.