Also, I would think the physics of an 800 lb animal rolling on a 50 lb child pose a greater risk than a 150lb adult.
Did the horse fall on top of the young child in that parade? So sad.
On the other hand, kids are remarkably resilient when taking falls, I got tossed a lot when I was young, and bounced right back on. As an adult, it hurts a lot more. A horse falling on you can be bad for anyone.
Aw hell, I've seen plenty of adults that couldn't control their horse any better than a small child.
Also, most, if not all, of the horses that you see these small children on in rodeos are 'bomb proof'. Most are 18 to 24 year old veterans of the rodeo circuit and if you take notice, you'll see that a lot of times these small kids are just on for the ride because the horse knows the pattern.
The world is a dangerous place and people die. Some sooner than others.