I like the Wii a lot, despite it's not the best console for me personally.
IMO it (and the PS2 to some extend) is the best console targeted at young kids. Most of us got introduced to computer games when we were kids.
What if there was no Wii? The PS3 is too expensive and multi-featured, kids usually won't get the most out of all those provided features. Sure there are some great family orientated games like Ratchet and LBP, but not many young kids have a HDTV and internet in their bedroom for example.
The 360 is cheap, but seriously that console is far too sensitive and failure prone for little kids to have to deal with. Lots of games like Halo 3 have limited value without having online and you even have to subscribe for that! (thinking of some little boys, I don't imagine little girls wanting to play Gear of War or Halo) So, IMO not a very kid friendly or kid targeted console as well.
If I would buy a console for my kids (which I don't have yet), it would most likely be a Wii!