Can Two Kids Share a Mac with Different Protections?

Absolutely! All you need to do is create a separate macOS user account for each child, and connect each of those user accounts to a Gertrude "Child". This allows each child to have their own individually customized sets of allowed websites and monitoring settings.

Just be sure that if one of your children has more permissive settings than the other, that their computer user is protected by a password the other child does not know.