They do not rely on bench-type substitutes. As a rule, there are few if any substitutes in most forms of sport for adults and many forms of sports for kids. Yet, your views are not unique Rush. They want to play. When denied this, they move on to other activities where they do get to play.
Youngsters do the same thing.
When they do not get to play in one sport, they move on to another sport. Unfortunately, some drop out of sports completely. This brings up an important point. When we rely on the bench to motivate youngsters to improve, only certain ones will respond to that challenge. Age or ability are inconsequential.