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HahahahahaTurn it off (hold the power button down, then swipe to shut down). Now here is the tricky part. Walk away with the iPad and put it out of reach. A safe could be a good idea depending on the situation.
Alternatively, you could change the passcode so that your child cannot access the iPad, then set it back during usable hours.
As you might have guessed, there is no way to "disable" an iPad based on tine of day. Probably the passcode idea is the closest thing, but not automatic. Physically removing the device from your child is the only other option.
Here is one vendor, the one I ran across: http://www.curbi.com