In reply to sarahkeast:
If a modern router you can often limit access and or bandwidth by IP address. You'd need to set the router to fix the IP address of each device laptop, Xbox etc. you do their by fixing he IP for thir MAC address. The MAC address is unique to a device , can be faked but I'm sure your kids aren't that advanced.
Old modems use usually only have limits on when they can access Internet. So you could do that to start with. Maybe only allow 6pm till 10pm during week.
Windows 7 could limit their bandwidth by user account , through the policy editor, but wouldn't deal with Xbox.