I don't know if JKR has ever mentioned it or not.But here is some info from the canon:

>“Attendance is now compulsory for every young witch and wizard,” he replied.
“That was announced yesterday. It’s a change, because it was never obligatory before. Of
course, nearly every witch and wizard in Britain has been educated at Hogwarts, but their
parents had the right to teach them at home or send them abroad if they preferred. This
way, Voldemort will have the whole Wizarding population under his eye from a young
age. And it’s also another way of weeding out Muggle-borns, because students must be
given Blood Status – meaning that they have proven to the Ministry that they are of
Wizard descent – before they are allowed to attend.”

>Harry felt sickened and angry: at this moment excited eleven-year-olds would be poring over stacks of newly purchased spellbooks, unware that they would never see Hogwarts, **perhaps never see their families again either.** -_The Bribe,The Deathly Hallows_

(Emphasis mine)

So we weren't explicitly told what would happen to them.We were just told that Harry felt angry about what was going to happen to these excited young children.

However,we can guess a bit. Note that there is no difference between a grown-up muggleborn and an eleven-year-old muggleborn. The following is an extract from the same chapter:

>".... said Lupin. “Nevertheless, unless you can prove that you have at least
one close Wizarding relative, you are now deemed to have obtained your magical power
illegally and must suffer the punishment.”

These muggleborns were not killed. Therefore there can't be a reason the eleven-year-olds have to be killed as well. So if they were not killed, and the worst that could have happened to them was that they were sentenced to Azkaban, then we can say/guess that in the year ensuing  Voldemort's death, they were once again invited to Hogwarts (I don't believe parents would want their children to go to Hogwarts after an year of not seeing them).

Again, this is just speculation.