Hello, I used to Autocross at about that age as well, and it’s perfect for kids especially, even me with how shy and anxious i used to be (Lol)
As Alan said, yes the Briggs and Stratton Animal or also even the LO206 (raptor) engines, are all very simple. You pretty much just have to pull start it and once its set up properly, there is really no mainenence. The clutches are usually bullet proof. We’d go atleast 3 or 4 years on the same chain and tires, doing literally no work to the kart other than some carb tuning here and there.
Definitly start out with one go kart, and since according to almost all SOLO rules, the karts all run in the same group, without other cars running. So what me, my dad, and later my sis used to do, we all drove the same go kart, it was a Briggs Raptor engine, and we had Zuess Tabs connecting a smaller seat inside the adult seat, as well as pedal extensions that were quick release. It was very goofy, but it worked! We had to do it efficiently so we didnt waste time, is the trick.
For the sake of simplicity, just have your son drive in the FJB class with your daughter. Depending on where your club is, usually they will allow for this because it isnt extremely competitive anyways. otherwise yes you will have to have your son race with the restrictor in FJA which isnt too bad. The issue we ran into was locally it was fun but nationally, I dont want to sound biased but the karters that do national autocrosses , theres a lot of cheating and pointless drama, over competetive.
The next step up from the 4 Strok Briggs is the allmighty Yamaha Kt100. Depending how your kids like it, thats a much bigger thrill, but a looooooot of maintenence. It’s a big step so hold off on the Yamaha for a couple of years atleast. We tried to compete Nationally with the yamahas once, and yeah people spend way too much money and its not fun. But these engines are for 13 or 14 years or older usually anyways. (starting with the 3 hold can class, then the 4 hole can class, then the pipe/adult class)
After a few years of autocrossing, depending where you live again, you may want to see if the kids are interested in wheel to wheel racing! It’s definitly more competitive and a little more expensive anually, (tires, fuel etc mainly), and hey im 21 years old now and im totally on my own fiddling with my karts, preparing to race Daytona next year. You can;t force the kids to like it but if they do like it, you’ll know real quick! :)
Karting, for me, taught me about driving at a young age yes, but also taught me a lot of science, mechanical skills, useful things like tire pressures, offset wheel bases depending on the AutoX course, etc etc. and it was the most fun i’ve had. You may not have the fastest go kart but hey its all about starting very small, testing waters, buying a complete 4 stroke kart for no more than $1000. Then from there, if the kids really love it, you can start teaching them those skills, just simple stuff as i said, but it goes a long ways. Affordable, fun, for adults and kids