Parental controls to disable WiFi and/or mobile internet during certain hours?

Outside of Enterprise and Educational environments where devices can be centrally managed using an MDM platform, your best option is to control network access from the network itself (such as your WiFi Router). Clearly, when away from home, access to other networks is principally controlled through knowledge of the associated WiFi Password (where applicable).

Your other option is to use ScreenTime controls of your children's devices. Information about available ScreenTime and Parental controls can be found here:

Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support

Use Screen Time on your iPhone, iPad or iPod touch – Apple Support

Set up Screen Time for a family member on iPad - Apple Support

Completely blocking access to WiFi is impractical. Consider that without a network connection, the associated device(s) won't be able to receive remotely applied changes to ScreenTime and Parental Controls; by design, many elements of these controls cannot be changed from the 'controlled" device that is signed-in to iCloud (again requiring active network connection) with the child's AppleID.