It looks like FJC is the class of choice (5-7 year olds)- that makes it pretty simple:
1) Be sure your region is approved to run FJC (just because they have FJA & FJB does not mean they are approved for FJC – probably are, but check)
2) Your engine choice is a Comer 50/51 – basically a big weed eater motor, no mods, simple clutch with no adjustments. Keep fresh fuel in it mixed with good 2 cycle oil and maybe once a year take off the muffler and clean out any buildup.
3) Maintenance – karts are basically trying to “disassemble themselves” whenever they are running – check all the nuts and bolts regularly
4) Read the FJ rules. Be sure you have the legal tires (Cadet or MG Red). Be sure you have the safety cable on the brake linkage. Safety wire/safety clip critical hardware.
Based on the way the rules are written today – look for a Briggs World Formula when the kid(s) are ready to move up to FJB. You’ll want a new “full size” chassis then anyway. Talk to karters in the Solo community, these karts get handed down as kids grow out of them. We have a FJC kart in the region that’s on its 3rd or 4th local owner and it has had at least 6 kids grow through it into other classes. Good luck.