I’ll second what Craig said. Comer 50 or 51 on a baby kart or a cadet-sized kart, depending on the size of your kid(s). The Comer is indestructible and the worst thing you’ll really ever need to worry about is a grain of sand in the carb jet. Well changing the clutch is worse, but you can get help for that. The kart chassis rattles itself to pieces, so safety wire and nylock nuts are definitely your friend. The axle gear is a spec size (89), so they can’t go much more than about 25 mph.