My boys (3 & 6) are big into starwars, so The Shwartz is their favorite. Only let them play this one while tucking them in for bed at night... where a drop will either land on them or the blankets (of course, I always remind them to "hold on tight so you don't drop it!")
My six year old also likes Robo Defense... It was tough (read: frustratingly difficult) at first, but I've been playing for a bit now and have most towers upgraded to almost 200% damage and reload. Now he can play level 0 and not worry about loosing as the rocket towers are basically one-shot-one-kill for most baddies. Abduction is another one he likes.
3yr old also likes "Poke A Mole" .. and "Zebra Paint". "Toddler Lock" isn't bad either.