Banjo Kazooie - Nuts and Bolts on the 360. My 5 year old would play it all the time if I let him. You need to play it yourself to unlock the various worlds and build a few interesting vehicles, but once that's done it's perfect - they can't die, and can drive, fly and float around destroying stuff and generally causing mayhem. The worlds are largely destructible, and the main character has this force power thing to pick up and move stuff.
I caught him lifting giant ants onto a big tractor I'd given him the other day, and carting them to the top of a volcano before flinging them in. Brilliant stuff
Viva Pinata, on the other hand, has a 3+ rating, but is actually far too hard for a little 'un to do anything but play the 'just for fun mode'. Requires a lot of reading up on creatures likes etc to actually play the game.