When you say chase I assume you mean Chassis, unless you go with an actual Kid Kart with a comer 50, which I don't recommend the chassis are same size as adult, exception Jr Sportsman champ karts, Are you thinking open flat kart chassis, or does Mom what a cage ? as far as motor look at your local tracks and see what they offer for his age group, most likely red plate clone.
Once you confirm with the tracks you would race at what they offer then go from there, you may want him in a cage but they may not offer that class, either way once you confirm I'd try to get 2012 or newer if it's in the budget, plus once you narrow it down to say 3 ask back here guy's will help more.
Good Luck !!