I rebuilt a dune buggy for my grandkids (7 and 9). Good running motor. Made sure the brakes worked, even with my 220lbs, and my 120lb wife are goofing get off in it. Put in 5 point harnesses, and my son supplied the full faced helmets.
Buggy will do 40 miles an hour. Had my wife clock me down the road, followed by a stern warning from the sheriff

...oops
Us big kids had to give it a thorough testing to make sure it was safe before letting the grandkids drive. We even tried to roll it, to judge where to govern it down so the kids get used to it. So, after me and the boys got done seeing who could do the best donut and had the most hang time We turned it over to the grand kiddos. Who then proceeded to give there grandma and momma heart attacks
Took those kids about 30 minutes running around the yard, before they figured out where the governor was. then all he$$ broke loose