How do I allow the use only of specific sites on chrome and firefox

The kids are getting on my nerv. I have to leave open access to the laptop and the apps on it. People in the family need to access a total of 3 sites on the inteet while I'm gone.

How do i only enable the sites i want and block *.* everything else?

All of the block mechanics I know rely on the blacklist concept. I need to make a whitelist and ditch everything else...