If all else fails you could use a short piece of small metal tubing to make a spacer that would slip over the throttle cable on the inside of the throttle spring located in the carb itself. It would take multiple trial and error fittings to get it right but when you find the spot you like, it would make avery effective throttle stop. Also would be highly unlikely to get monkeyed with by the kids. I know when i was a kidthe screw in the thumb throttle only worked on mine for one ride. I found that screw pretty quick. Lol!
I was born and raised on Venus & I may be here a while.....