i use rhino and i have tried my 10 year old on tinkercad, 132d and a few others but he asks for my help and i cant help him because i just find those entry lvl cad programs frustrating and actually more difficult because of the lack of tools and functionality. i would have your kid start on 123d design atleast for a couple few weeks though just to get his feet wet. rhino is great and really is super easy. it can look very complicated especially to a little guy because of the tons of buttons but you only us 20% of them...normally. i am going to go and set up custom modified tool bars and eliminate all the unused stuff and make a work environment for my kid. just put all the things out that he will use. then if he needs something thats not visible you can just type into the command line. its a good way to a young person to use a professional and powerful cad package without getting overwhelmed and as he grows with it you can bring out new tools and functions.

btw, if you have a mac you can download the mac version for free. its in development but pretty much completely functional now. i like the mac version better now.