If you really believe it is a cult, then I would not let any child be alone with them. Visits to grandma and grandpa only if mom and dad are there too. Don't underestimate the influence of the teachings at the KH under the guise of someone they love and trust.
Most JW grandparents I knew that had children who weren't active JWs, made plans as to how they could influence their grandchildren to become JWs...after all they believe that all non-JWs are going to die. It has to be the right thing to do, to save their life!?
Blondie