I could never do that. It's like when I see people carrying a baby on a half wild horse on a trail. I watched a horse rearing up with a baby on it one day (like 5 month old baby). My little brother started on a pony that ended up not working out. Unless you rode him for at least 2 hours a day he was a bucker. :( He is now on a TB, but its a solid horse that has noo problem with getting next to anything to be mounted. He handles the horse by himself almost completely. If I didn't think a child could stay on without something like that I would not want to have the child doing it at all.
My friends and I were riding fast at 7 and 8, but we knew how to stay on. When I fell off I asked for pointers on how not to do it next time. The only time I could see using that velcro is with therapeutic riding because those horses are solid, have multiple people watching them, and are controlled.