What I have heard(but never done myself) is drill a hole in the top of the carb above the throttle slide and thread it for a small long bolt. Then you can just screw the bolt in to restrict he throttle slide and restrict how far he can open the throttle. As you and he become more comfy you can just turn that bolt out to give him more and more power.I am busy with the same exercise for my 3 year old, he’s out grown the 12.5. I am ruling the 50 sx out. He climbed on one and pinned it, those little 50’s move. His life flashed in front of me. Lol. He loved it. Me not so much. I’ll wait till he’s a bit older before we go that route. Haha. Good luck.