You could try something like this:-
If your rooms can be fed from Ethernet ports and you intend to use Ethernet to "feed" a WAP (wireless access point) in the kids rooms you could put an Ethernet switch in between the hub and the Ethernet patch panel that feeds the kids bedrooms.
Because the Ethernet switch needs to be plugged into an electrical socket it means you could put the switch on a timer.
This would allow you to turn the Ethernet switch off and on at your chosen times which in turn would disconnect the kids devices from WAP and obviously the Internet.
You would off course need to ensure that the kids do not have access to the timer or be able to connect to your home wireless network that is being transmitted through out the house which may spill into their room.
