I personally don't see a difference between the Xbox and ps3 except that Xbox is cheaper (for me). I love the Xbox for all my violent needs piratein other words I find that it's much more common to see first person shooter games and other mostly violent games. ps3 is the same way and in this case covers the same area. I love the wii for another reason, family friendly games such as Mario Party, wii sports, etc. Overall I think my favorite has to be wii for it's general genre.
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.