Family online time management

Hi

 

I am run Norton 2010 and online family on 3 computers I have time limited the kids on the computers to one hour and half, my oldest son has found that if he uses his time on 1 computer he can go to one of the other computers and get a other hour and half and so on. Is there a way that we can stop this for happening or is that just the way it is?

 

Cheers

Chris