Gravity is a thing and yes your child will experience equine assisted gravity checks.
Falling is part of riding. As much as we try to not fall it will happen. You must balance on top of the horse when riding. There are times the horse may trip or spook (move quickly) and this may cause your rider to fall off. They may lose their balance while taking a turn or going over a jump. They will fall and hopefully with no big injuries.
There are ways to help protect them during a fall. They can wear a helmet (required by all barns), safety vest (padded impact vest), or an air vest (blows up during fall). All of these will help protect them during a fall but will not prevent the fall. Heels down will help with prevention.
So yes they will fall and it will be more than once. All we can do is our best to help prevent injury.