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.
We have run every plate under the sun for a clone. We can turn whatever you want them too based on gear. We have turn red plate 6200 before. I have found to gear the young starters out to flat foot it. I teaches them to control kart before they have to worry about lifting. Most cases they will be faster with less speed.
we used to run the 5-8's on a purple 8-12 blue and 12 up unrestricted and they were way to fast . 7-9 i'd do green 9-12 purple. gear rules really hurt kids in that upper age group since you have kids can really grow at that age.so unless you run a really heavy weight that's going to put big kids at a real disadvantage