What's your favorite mom trick for getting kids to wash their hands? Your answer may be used in an upcoming story!
Just be consistent. If you have to make them go back EVERY time and wash, they will learn that it is just easier to wash before coming out. You can also insist that they tell you they are washing, or they have to wash again - that eliminates the guess work - did I hear the water in the sink over the toilet refilling?
Help to have a happy family.