Let us look at the syntax tree of
The children either forgot their shoes, mittens, or hats.
{Entity} either {
list-of-choices
)
Where list-of-choices is exemplified by
{Entity} either {
{choice 1},
{choice 2},
{choice 3}, ...
or {choice m},
)
The principle is that each choice should be able to form a well-formed statement with the entity:
Either {
{entity}{choice 1},
{entity}{choice 2},
{entity}{choice 3}, ...
or {entity}{choice m}
}
i.e.,
{The children} either {
forgot their shoes,
mittens,
or hats
)
should be transformable into
Either {
{the children}{forgot their shoes},
{the children}{mittens},
or {the children}{hats},
}
Therefore, a more precise way to say it would be
{The children} forgot either {
their shoes,
their mittens,
or their hat
)
because the following transform would comprise well-formed statements:
Either {
{the children}{forgot their shoes},
{the children}{forgot their mittens},
or {the children}{forgot their hat}
}
Which is still not precise enough because there are only three children and only three choices where each choice is exclusive to a child. i.e. there is a one-to-one relationship between the deficiency and a child.
The following is more precise
{Each child} forgot either {
his or her shoes,
his or her mittens,
or his or her hat
)
However, the English language is very forgiving where clarity takes precedence over syntax.
{Each child} forgot either {
their shoes,
mittens,
or hat
)
If the pronoun "their" is allowed to elide into the choice list, then why not the the adv/adj "either"?
Therefore, the sentence
The children either forgot their shoes, mittens, or hats.
is not as syntactically logical but it does present a measure of clarity and brevity, as well as a provocatively annoying variance of style.
I would say, go for it. Let the language evolve.