Here's an "out of the box" solution... Why not use an aquarium timer? I have my two tanks set up on a timer that turns my lights on and off at certain times. You can connect one of those to your Base Station and have it shut off altogether between 10PM and 4AM.
while your 'out-of-box' workaround would certainly get the job done, skorpien, it isn't exactly the most
elegant solution (which reminds me... is there a graceful way to 'shutdown' the AirPort/TimeCapsule - other than unceremoniously pulling the plug?).
anyway, about the Timed Access thing, it would be better for Apple to fix what is an obvious bug or oversight in the firmware. the time restrictions should apply to both networks, either concurrently or independent of each other.
of course, there are situations when 24hr access to the public network (internet) is needed (while restricting access to the private network), so in that case, maybe a new 'checkbox' under the Guest Network tab, that toggles 'Impose Access Control Restrictions' would be a simple solution.
btw, that's a brilliant idea, Allan. i ought to setup time restrictions for my OWN internet usage, then i might get to bed on time myself!
btw, unless you've configured Access Control for 'No Access' by default, then it would be easy for anyone, your son let's say, to spoof (change) his MAC address... and bypass the time restrictions.