I worked as a 911 operator years ago. There were a lot of children who would call in. It is not the worst thing that can happen, as it can be a valuable learning experience for the child if the parent follows up the call with talking to their child about the use of 911.
The problem with calling and hanging up is that if there is no answer when the operator calls back we are required to dispatch an officer to the location. The assumption when there is no response is that there is either a health emergency, home invasion or other emergency that required the caller to disconnect.
It certainly is a draw on police resources if a child were to call over and over again. It ties up the line when there may be a dire emergency. There is certainly room for mistakes, but it's important for parents to help kids learn about 911.