As far as I know, there isn't a way to disable using Khan Academy at home other than having their parents block Khan Academy on their browsers at home. I don't understand why you would want them to not use it at home because if they use it at home, it means that they have a desire to learn, even when they aren't at school.

It would take a while to tell you why I would like this, but there are several very legitimate reasons.  I think I may have figured out a system that will work for me.  Thanks anyway.