It would be more practical to make one time limit for a child that would be applied to any machine the child uses. Now if a child uses several Windows machines, you will have to set up a separate time limit for each machine. This is not practical at all. And if you want to change the screen time limit, you will have to go through each child's profile and set it for each machine in his profile.
I have many children and several PC's, and I see how difficult and time consuming it is. Setting one time limit for a day is much more useful. The child can use any machine, but the time he spends on each machine will be counted towards the one time limit.