[jthomas1600]

OK. So I've got no real experience with this. My daughter bought a bass a little bit ago and she already thinks she's a rock star apparently. She's having "band practice" tomorrow. I think there's a kid who plays guitar and a kid who sings and I'm thinking I'll put some drums in the garage when I get home. My other two kids will like that, we've talked about it before. Anyway, obviously you get a couple of teenage kids making a racket with an electric guitar, a bass, and a drum set it'll be pretty hard for the singer to keep up. So....

what's the cheapest, simplest (there's are just 14 year old kids) way to amplify vocals for a garage band practice type scenario?

[Rainer.]

Any full range amp/PA would be best.

But just get one of these:
Amazon.com: XLR Female To 1/4" Male Adapter: Electronics

And plug the mic into whatever amp you got. Might not sound pretty, but it'll work.