Good call George, we are doing the same. Possibly just like Old Goat's(Jim Klauss) daughter's, I think is 10 to 1 and 5" rear rim with 11" Kart Tire.
The throttle is set by the parents, And the gas pedal engages the belt. We might even use a push mower engine. (6.5hp)
Getting ready to build a prototype...hopefully to match yours and Jim's rules so the kids can race with each other.
The kids are our future!
Florida has something called a Mini Mow, but it is stick built, nothing is mower on it at all, looks like a miniature FX-T Go Kart.