I don't think there is any way to do this on. The iPads are doing stuff in the background, i.e. checking email, perhaps connecting to Apple servers checking for software updates, etc. So from a router point of view, it would probably be extremely difficult to tell if somebody is really using it and start counting "time" against the 2 hour limit. I haven't seen this feature on any router at all.
Now what I have seen on some routers is an option to set a time schedule, i.e. you can block certains clients (the kids devices) from going online at times, i.e. monday-thursday you can allow internet only for 2 hours after school, and then block it for the rest of the night, Weekends, you allow it for a bit in the morning, then block afternoon, etc..
Unforutnately Netgear Orbi does not support the second option I listed above. I hope they add that feature. What I am considering doing is changing the Orbi from router mode to access point mode and putting a router behind it (i.e. some generic router, which I will run tomato or dd-wrt on) which will allow me to block the kids devices from the internet.
