1 kid, 1 choir = kid’s choir

multiple kids, 1 choir = kid’s choir

1 kid, multiple choirs = kid’s choirs

multiple kids, multiple choirs = kids’ choirs

If, in the extremely unlikely event there were only one child in the choirs, it would be kid’s choirs, this would also be the case if one child owned the choirs. There is no apostrophe needed for the choir as this is not possessive.

I guess if I had my ‘druthers’ I would put “Adults’ and Kids’ Choirs.”

Or say “Adult and Kid Choirs.” Keep them the same. That way that single adult doesn’t stick out like that, and each group has a choir. I hope that makes as much sense to you as it does to me.

OR

adults’ choir and kids’ choir

if ya wanna be anal about it…and I do!!!