Tricky Tricky. I DID buy my son (when 9 and 10 in 2 weeks) a new Yidilz .410 over and under last year. He has shot with it but it's still a bit long. I'm afraid I did fall foul of trying to get him shooting a wee bit early. The problem is that his shooting is so variable given 1) it's a .410 and 2) he's small and it's a bit big for him. So one has to coach carefully or they become disillusioned with missing all the time. Don't get me wrong he loves it but I think a 20 when he is a bit older is probably more appropriate...
Just my own experience!
T