Welcome to the forum. The short answer is no you can't, there's no option to restrict access to certain devices by schedule like some routers have. You'd need to look at any control options available on your child's device itself or any software/apps that can be used on the device to achieve the same thing.
The long answer is it should be possible to achieve this rather inelegantly by allocating a fixed IP address in the router to your child,s device, then setting a custom schedule and then setting a firewall rule to block on schedule all outbound communication, and finally allocate that firewall rule to the IP address in question. In reality this doesn't seem to work reliably or at all on the NOW TV Hub.
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