Teaching kids the necessary manners inside the lavatory can be a tough task. You need to teach them to not only get done with business properly, but also to use as little resources as possible. Exhausting resources like toilet paper and water in the lavatory can mean that you will have to face the brunt later on.
One way you can keep a check on the toilet paper being used by your children in the lavatory is to have a marker made on the wall. Children often end up using more toilet paper than necessary because they don't know what is the sufficient amount required. They just keep tearing away with hopes of getting the right amount. This marker would clear away their confusion, and will save you some toilet paper as well for future use.