I've been in a number of plays (or involved with them) and I can suggest a few.
First, there is a play called "Moonchildren" (it was originally called "Cancer"). THe play is about a group of fourth year university students in the 70's. Pretty good, and it's realistic. SImple setup with plenty of laughs so the kiddies should enjoy it. Not hard to put on either. A group of people I know put it on wiht a bunch of props they had lying around their house.
THe other play I'd suggest is "Baron Bollegrew" (I think I spelled that correctly). It's a play about a group of knights who are bored, so they send one of their number off to an island to take care of a despot. Funny, loads of laughs, and again, simple to put on. It even has a dragon. Think something along the lines of a "Nodwick" or "Knights of the Dinner Table" comic and you've got Baron Bollegrew. Oh yeah, throw in bad brit accents, why? because you can.
Good luck with this. Oh yeah, have fun to.
EDIT: Just noticed your cast size. Moonchildren only has about 16 parts. And those are mostly guys. So, you can't really use it unless you use other plays with larger casts.