Which is the best Linux distro for kids under 10?

Have you considered PiperKit? I'm not affiliated in any way, by the way - this caught my eye because it seems like you're trying to get a young one interested in software (kudos). I used the Play Piper kit to teach twenty 7-8 year olds in a summer camp a couple years ago, and it worked great. We wrote Python code and built "smart" doll houses out of cardboard with LEDs for room lights and computer fans for climate control.

Linux mint is really nice. I havn't touched it in a while, but it has everything as in codecs, easy to work with. Nothing fancy. Also the UI is great