If it's little kids, I say the Wii. If it's older kids, then the PS3. Huge bonuses of the PS3 ... it can go online (it has a built in web browser, IM feature, access to the Sony store, etc - for the XBox, to go online, you must have an XBox Live account, which costs money, the Sony online account does not), has wireless controllers included (XBox doesn't, you have to purchase them seperately) , and it doubles as a BluRay DVD player. It gets far more use in our house than either the Wii or the XBox 360.
Yes, we have all three systems. lol We keep the PS3 downstairs with our 41" tv and stereo components, for hubby, myself and our teenage son and all his friends. The XBox 360 and the Wii are upstairs, on the kids TV, although we all play these systems as well.
I will say that there are far more "kid" games for the Wii than the others, but it is always a changing market.