Something belonging to a group of children (= children's surely?)

[idril]

Year 4 son had homework on possessives.

He had to write how you show "something belonging to a group of children". He wrote "children's" but it has been marked incorrectly and replaced with

" childrens' "

Am I going mad or is the TA (homework marked by TA) wrong?

I think you're right.

Students' union

but

Children's centre

Yes you're right children is already plural so the possessive is children's

Your son is correct. Children's.

So men's toilet vs ladies' toilet
...I assume...

Thanks. I thought he was right but it's so easy to start doubting yourself.