How to keep kids off of our retaining wall?

How high is the retaining wall? Is it a privacy wall or is it there to level the back yard? The reason I ask is because everywhere I've lived there has been a fence spec with a minimum height requirement around swimming pools. So you may be out of compliance there. You are setting yourself up for a lawsuit if one of the kids falls off the wall and gets hurt or if they get into the pool and get hurt. In the meantime the best thing to do is call the police and let them handle it. You might also take some pictures to help the police when they contact the parents.