Pick N roll or Moving Screen

[TigerBball]

I coach 7th grade boys and whenever I teach a screen and roll, one of them always questions whether moving after contact is legal.

I tell them if they are pivoting and using their backside to contact the defender, they are OK, but if they keep facing the defender and move both their feet to keep him pinned, it is going to be a foul.

My problem, this explanation is just from my experience. How do all of you differentiate between a screen and roll, and a moving pick.

Just trying to teach my boys the proper way, I appreciate the help.