I bought a PS3 + Move for my 4/6 year old (at the time) to replace the Wii (I wanted a decent console in the living room for myself)..
All I can say is, it's rubbish for young kids, all we found where the silly party games that they got bored with in 5 seconds flat, all the 'sports' type things (they where fine on Wii Sports and other Wii games) are far too serious for Move, they couldn't get on with them at all.. They just don't cater for young kids at all.
Obviously every child is different, I've never come accross a young kid that can't play kinect because they have to hold something, but if that was the case, then a Wii is what he needs.. cheap, easy waggle games, perfectly suited to the most casual of casual..
We ended up getting a kinect + 360 for them, and they've really taken to it, the fact it has no controller (in contrast to your own child) has meant more games the youngest can engage with. We've sold the Wii as it was gathering dust, and will be getting Sports Season 2 and Disney Adventures for the kids for Christmas..
I thought Move would be better, I conned myself into making up reasons the kids/missus would like it, but the reality is quite different, I'd honestly stick with the kinect, if you want a PS3, get one for yourself (and I'd probably ignore Move unless you absolutely are sure you like playing motion games)..