...I did that stuff all the time when I was a kid/young teen. There are worse things in the world than desensitizing with blankets, or kids pretending to fall off. I fondly remember my riding buddies and I riding by and falling off into fresh shavings, and my one friend trying her best to spook her horse, who always gave her the same "really? Better luck next time..." expression.
Harmless fun, and I don't see anything wrong with goofing off as long as both human and horse are safe. Someone wants to pretend to fall off? Fine by me... if they do it wrong, they'll sprain something. It isn't hurting the horse, and they'll learn if they do it wrong.