Look: I have 3 Children, Child 1, Child 2, Child 3.
I also have 2 PCs.
So I think it makes sense to make one time limit for each child irrespective of the machine he uses.
So I might want to allocate 4 hours of PC usage for Child 1, 6 hours for Child 2, and 2 hours for Child 3. During the day they use whatever PC is free.
So Child 1, for example, might use PC 1 for 2 hours. In that case he would have 2 hours remaining regardless of which PC he uses.
I mentioned Android, since some children might have phones and tablets. So phone and tablet usage shouldn't be mixed with PC usage, in my opinion.
Or you could make an option to allow user which device usage would be aggregated towards a common limit and which ones would be calculated separately.