If  your young piano player is having difficulty with a particular spot or measure in a song,  have him/her slow down a little right before it and make sure its played correctly.  Then have him/her play it again a little faster until he/she can play it correctly without slowing first. A young piano student should never stop completely as that breaks the flow of the song and teaches the muscle memory to pause or stop.  Also,  ask him/her not to go back to the beginning and start over each time.  The pianist should only go back a measure before the problem measure.  Studies of piano practice methods have proven this to be the best way to progress into a pianist with lovely flowing technique.