Some wifi base stations have access controls. They are a pain to manage and work differently depending on brand. But, the basics are that you have to identify each computer or other wifi device by it's WIFI address (it will look something like this: 29:db:e9:14:75:cf) and then set a schedule for it. You do all of that on the wifi base station which you generally manage by connecting to it with a web browser. If you happen to have an Apple Airport, I can explain in more detail since I know those well.
Another option that parents might consider is OpenDNS -
https://www.opendns.com - which is free for home use. You use it to block categories of websites to block. It doesn't have schedules so it's not a good option for the OP's request. I set the kids' devices to use OpenDNS while we parents use the normal DNS and, therefore, are not subject to the same blocks.