The seed in the parable is the word of God, that is, the message. Compare this parable to the Parable of the Sower which occurs immediately above this parable in the same chapter (Matthew 13) and includes an explanation of the parable.
Matt 13:3-4 And he spake many things unto them in parables, saying, Behold, a sower went forth to sow; And when he sowed, some seeds fell by the way side, and the fowls came and devoured them up:
And now the explanation:
Matt 13:18-19 Hear ye therefore the [explanation of the] parable of the sower. When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side.
The seed is the word, as is even more explicitly stated in Luke's account of the parable, in
Luke 8:11 Now the parable is this: The seed is the word of God.
The Parable of the Sower is the foundation, the key, for understanding the other parables, per
Mark 4:13 And he said unto them, Know [i.e. understand] ye not this parable? and how then will ye know [i.e. understand] all parables?
So then in the Parable of the Tares, the seed is still the word of God. The field is the world (as the explanation tells us) and the good seed is the word of God, while the bad seed is the word of the devil, and these two words produce children of God and children of the devil, when people believe them. So that those who believe God's word become children of God, and those who believe the devil's word become children of the devil.
This is explained in
Matthew 13:37-39 He answered and said unto them, He that soweth the good seed is the Son of man; The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one; The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels.
One point of confusion that one might encounter is it sounds like he's calling the people the seed rather than the word which they believed. But this is easily overcome by comparing it to the Parable of the Sower. In other words, the phrase "the good seed are the children of the kingdom" means "that which is produced by the good seed is the children of the kingdom."
(All scripture quotations are from the KJV)