When I was 8, my dad bought me a Honda ATC 70 three wheeler. Got pulled over by the cops my first time at age 9 for bombing down the (rural, dead end loop) road on it. Always wore a helmet, somehow survived. After a couple years rode an ATC 110. Wasn't the fastest thing out there, but I'd love to have one to haul stuff around the yard today (it would pull pretty well in low range).
I have two boys now, ages six and four, and their granddad bought them a used 120cc snowmobile to ride at his house. I had to fiddle with it a bit because it had sat for a couple years, but it runs really well now. We started both the boys with the tether switch connected to a long dog lead. Somebody would stand in the middle of the yard and the boys would do laps until you were dizzy. After the blizzard this winter, it was all we could do to pack a trail for them with the adult sleds, when we tried the leash, it would just get stuck in the deep snow...so now they're off the leash. My older son is more cautious...he wouldn't even ride it at first without the leash, but my younger son did, and he crashed the sled at 2 mph into the house.
The sled is governed down to about 8 mph...I have the stuff to make it go much faster, but I tried it myself and put it back to stock very quickly. My older son is one to slowly explore the limits of a machine/activity and work his way up to doing something...my younger son likes to try the difficult thing over and over until he figures it out. Both of them got reign over the trails at their granddad's house, and I put 50+ miles on my sled following them around the yard this winter...at 8 mph. They have full-face helmets and now wear the engine kill lanyard around their wrists.
We ski a lot as well, my older son started at three and my younger at two. Both of them will likely be able out ski me next year, they can go anywhere on our local hill (but they have a favorite run that I've done about 500 times.......please can we go on another trail?). Funny thing that happened this year was we shared a chair with a ski patroller once and we were telling him how we were skiing in the woods (glades) at the area and he was giving the boys high fives and encouraging them. Then we get off the chair and the boys go into the woods and come out where there is a mini "cliff" about 8 feet above the trail. My younger son picked the easiest way down it and took off, my older son was debating taking the more challenging way down, which I KNEW he could do, so I was coaching him on how to tackle the hill. He decides to take the easier way down and another patroller skis over and gives me grief about making sure my son skis within his skill level. My boy had just decided NOT to try something new (that he could do, and has since), and I'm getting lip about how my son skis.
Where am I going with this? Each child has his or her limits, as a parent you have to watch what they are and figure out the best course of action for each child. When I was at the ski area, I met a SKI patroller on the lift, who was encouraging my sons to try new things and grow as skiers...then not ten minutes later I met a ski PATROLLER who had to be grouchy and condescending and trying to put people in what he saw as their place.
If you're tuned into your kids' strengths and weaknesses, you're doing fine, maybe not perfect, but perfect is the enemy of the good. Sounds like Whitespider has things under control.