my son will be 6 in October. He tried his buddies Chinese 50 cc quad and loved it. I didn't want to buy a cheap Chinese one, so I found him a raptor 80 its an 06 3 speed semi auto.
My wife thought I was nuts but I set the throttle limiting screw and I start him in second gear so he doesn't rev the hell out of the machine in first gear. (he doesn't know how to shift yet)
He cant quite squeeze the hand brakes, but he is good with the foot brake on the right side. He follows me down a nice straight gravel trail and will also ride in a large open gravel pit. He picked it up really quick.
He now wants a dirt bike... but to get back to your point, my thinking was the same. Get him into the sport on something stable that has 4 wheels. once he has the throttle and braking concept we will switch him to a dirt bike.
P.s make sure he has the right riding gear. Our little guy has small youth helmet, riding boots, shin guards elbow pads and a chest protector.