There are two things on the KFX70 that keeps the speed down. The first is the adjustable throttle stop and the second is the torque converter belt drive is set up so the primary drive pulley sheaves (the guy with the kickstarter dog attached to the outside end) will not close enough to force the belt to the outside edge and in full top gear.
Take a look at a parts diagram for the clutch and you will see a set of spacers that keep the sheaves from closing and forcing the belt into high gear. You can use a caliper to measure your belt width, then use trial and error to remove enough of the spacer to allow the pully to close so the belt is all the way to the top.
Once you get it shifting fully and the throttle to open, increase the main jet by one full size and move the primary needle up two grooves. Increase the pilot jet by one size, and you will get a lot more snap off the line.
Your new top speed will be around 40 mph +- 4 mph, so get the kid a Simpson neck brace and show him/her how to wear it along with their elbow and knee armor and a first rate helmet without a visor (the visor can flip their neck back when they crash). By the way, it will pull you around at that speed too!
Some guys pull one or more of the thin shims off the secondary pulley, but that allows the machine to shift up too fast and it will lose usable power - it is like going to high gear too soon.