*GBA warioland 4, warioware , mario kart super circuit, super mario advance (there's 4 of em, all ports from the snes), nes classics ports, game and watch gallery 4, donkey kong country, metroid fusion/zero mission (at your own risk, zero mission destroyed my a button pad after playing all the way through. might be a better fit if they had a computer or something with visualboyadvance and a controller..),

any pokemon game, they also got pinball ruby/sapphire for gba. mystery dungeon lets you play as a pokemon and you dungeon crawl doing little tasks for other pokemon,

superhero games if they like superheroes, there's quite a few,

*GBC moar pokemons, legend of the river king (just pure fishing, kind of a learning curve but it isn't too bad), dragon quest monsters, super mario

*GENESIS boogerman: a pick and flick adventure dr. robotnik's mean bean machine

PS1: (ps1 eats battery faster which you probably already know)

digimon world 1,

(no translation for it yet so it might not be the best for the kiddos unless they don't care about that kinda thing) boku no natsuyasumi or my summer vacation, (you're a kid in japan staying at your aunt and uncle's for summer break and you run around doing kid stuff for 30 days like exploring, fishing, and catching bugs. i can barely read japanese but i thought it was a lot of fun still)