The young shooter's can have trouble holding on and if the rifle gets away from them after a shot, its just like dropping it before the shot is taken except always worse, the safety is off and its loaded and ready to go.
An Ar in .223 ain't getting away from nobody, even from a boy of small stature in the picture. I have never heard of anyone having trouble holding on to one either. There is a reason it is the most popular rifle in America today. Its one of the easiest for anyone to shoot.