The PS2 can play PS1 games. That's one of its biggest selling points. However, you can probably get a PS1 at a place like Gamestop for next to nothing these days. I warn you that your 12-year-old will not exactly be the envy of his teenage friends if you buy him a PS1 rather than a PS2, though. 😛
I personally think the GCN (Gamecube) is the best of the three next-gen consoles, and it has quite a few kids' games, as well as plenty of titles for more mature players. It's also cheaper than the PS2 or XBox.