The best way to protect your kids from ideas/beliefs that you don't agree with is to teach them to think. If they learn to reason through a problem or use logic to evaluate a statement, they'll never be swayed by opinions- even those held by grandparents they love.
If it's at all possible to make a relationship work with their grandparents, I'd try. The more people in the world who love our kids, the better. Plus, banning the grandparents might backfire later on.
Kids have a way of finding banned things irresistable, and they will probably find a way to communicate with their grandparents just because you don't want them to :)
If that happens, you won't be aware of what the conversations are, and you won't be able to help your kids process and reason through what the grandparents told them.
The Covid stuff though- that falls under the same rules as seatbelt and helmet use. Your word is law. What you decide is what they need to follow. Period.