I have a small whiteboard with an erasable pen and eraser on the end of it by the door. When a child needs to use the restroom he/she must print his/her name on the whiteboard before leaving the classroom and erase his/her name when returning to class. I allow only one student in the restroom at a time. Students know that when a name is on the whiteboard they need to wait until that person returns. I like having the child's name on the whiteboard so I know exactly who is out of the classroom.