So, the wife and I have a proposal out to the city Parks & Rec department to start a theatre program through them for this upcoming Spring season.  We're getting down to brass tacks, now -- a solid write-up for the upcoming catalog including a show name.  Due to budgetary constraints (we need to get the whole thing on for $1000 or less, more or less) it needs to be a straight play (also, we don't have musician personnel readily available).  However, we don't really know what kind of response we're going to get, so something with a flexible cast size allowing for both adults and children would be ideal.  Nothing too edgy, let's not freak the norms here the first time out.  :)

Your safest bet is one of the predictable, overdone "popular" comedies... (i.e.: Neil Simon)... which also have pretty high royalty rates.
I'd suggest going for a collection of short plays - audiences tend to be more willing to take a risk on a bunch of ten minute plays; every actor gets to play a lead role, and rehearsals when you only need two or three actors at any given time tend to be easier to schedule!
We did a collection all on a theme, ("love plays on") all by the same playwright at our community theatre - the audience loved it & we got to cast just about everyone that showed (including the kids), and have a couple of outstanding actors play 2 or 3 roles in the show.

I love Simon, and I've been trying to direct Brighton for a couple years, as soon as I can get one of the other local companies to schedule it, but I can't think of which of his comedies has a largeish, flexible cast size?

Simon's "The Good Doctor" is worth looking at, too. A series of sketches based on Chekov short stories - very flexible cast, although you'll run into the "costume" thing again.

You've probably heard me suggest this before, but I'd plug The Brothers Grimm Spectaculathon. Cast of 5 to 40, flexible costume requirements and easy set - it's been produced at about 40-50 community theatres. It's kind of like the Complete Works of Shakespeare (abridged) except with the fairy tales of the Brothers Grimm and more PG. It's been done in lots of middle schools as well, so I could see some younger children in some small roles.