I have occasionally wanted to trigger location-specific behavior on my kid's devices, like turning the wifi off when in their bedroom. This feels broadly in the category to me.
Couldn't you do this with your router/firewall? Of course, this would mean managing devices by MAC instead of DHCP, but one of the wifi routers (maybe a Tomato upgrade) allowed designating blocks of time to allow/deny WAN access
Doesn't that mean you'd also need to somehow disable mobile data, and possibly calls as well if you don't want them phoning their friends late at night?
Might be easier to insist that phones charge overnight in some parentally-observable central area... Though I suppose they might use them for alarm clocks.