mini genesis 6-12 pounds, or genesis 10-20, is defiantely the best way to go for a kid just starting out. one of the main factors is how much weight can the COMFORTABLY pull and hold? if a 20 pound bow is easy for them, then you can look into a bow that can eventually be cranked up to hunting weights. if they can't handle 20 pounds, then start them on a genesis.
i've seen quite a few kids at shoots, that almost dislocate their backs trying to saw back on their bows. the key is to keep them at weights they can control comfortably