Jetting a clone really doesn't control the speed as much as using gearing and let the motor run. Using a 12 tooth driver to a 64 tooth sprocket with a 34" rr tire will give you 35 mph max. Allowing the stock motor to freely will not hurt it. Started my grandson will this setup for 1 summer at practice sessions & changed the gear ratio as he got better handling the kart.
Being careful with the youngsters allows them the opportunity to develop their skill at driving at lower speeds while taking away the chances of getting scared & not wanting to race karts again.
It will still be competitive with others using the same setups. Some tracks do have a gear ratio limit for young racers - where they allow a specific driver & 3 different sprockets to cover different situations.