I'd suggest the medieval Mystery Plays or Passion Plays... The "York plays" (the ones performed in York, England) are the most commonly used today. I don't know what publication of these is a good text, as I've only read selections and saw them performed in England, but here's my argument for them:
They're the Judeo-Christian bible tales, which won't likely upset parents in rural America.
They were made for casts of dozens.
They were written to be appreciated by common farmers and performed by members of trade guilds, so they aren't elaborate or complex.
They are really good theater telling the biblical tales from creation to doomsday as performed from the 13th-15th century. They are mostly told through small plays with four people, so you give each of your kids a chance to star in a vignette.
They're really beautiful, touching plays at times (Abraham & Isaac is in every drama lit textbook as one of the best English plays ever) and very funny in others (Cain and Abel is actually quite funny).
In York (and in modern performances), they were performed over several days, so you could get 2 or 3 of your plays out of them.
You might need to do a job of adapting the language to modernize it. I'm betting there's a modern language version out there. There's probably even a version specifically for kids. If you can't find one but you'd like to do them, email me -- I'd love to take on the project of modernizing them for kids.
Here's a copy of some of the plays from the York cycle:
http://tinyurl.com/9gsx
If this is too elaborate, someone's suggestion of "You Can't Take it With You," is a good one -- lots of childrens' theaters do it; it's wholesome fun. So's "I Remember Mama," though not the best for the demographics of your kids. There are lots of fantastic African-American plays out there too, though I'm not really well-versed on stuff for kids. Having them create one themselves is also a great idea, and something kids really respond to.
If you're having terrible trouble with this, email me -- my brother-in-law does children's theater education and can likely come up with a good list of stuff for you.
Aaron