There is no possible way that "making karate videos" is an essential activity, so I am already questioning his judgment. If he thinks that is a reasonable excuse for contact with other people, you can pretty much guarantee that he isn't taking proper precautions in other areas of his life either.

"Choosing your circles" is not a thing. Imagine the walls of your home and the people who sleep inside. That is your circle. Ta-da! My (otherwise very intelligent) sister-in-law said something similar as an excuse for regularly hosting two non-household members. No. Stay home. I know it sucks. It sucks for everybody. But so does inadvertently infecting other people and killing them.

Do you have to let the boys' father see them? (i.e., is there a court order to consider here?) If not, I think the smart thing to do as a parent is to keep your boys in your house and away from him until this is over, while encouraging video or phone contact.