Well, here we go. I'd have to choose wii, mainly because it's got so many games that you can invite your mates over and play with them. Sure, 360 has halo which was fun for a while, and PS3 has little big planet but still, they just don't have that good variety of non-blood and bones action games. Wii does have things like super smash bros, super mario etc so you could invite your friend and their little brother, without brainwashing him with guts and gore
TL,DR: Wii as it's easier to introduce little ones to gaming easier. They can move onto xbox and play station when they're older.