What an interesting dilemma. Careful not to be hypocritical. Many posters on this site villify JW family members that presume to override parental upbringing by non-JW parents of their children and who try to "indoctrinate" those children with "the truth". In reality these JW family members are doing what they truly feel to be right and moral in their heart. The same applies in the reverse case as outlined here.
Both sets of situations involve a non-parent family member seeing the child "being endangered" as if they were about to run out in the street for a ball. In such a case, your expectation would be that someone care enough to keep the child out of harms way.
It seems to me that in this particular situation that the grandparent has to respect the parents right and responsibility to raise their child as they see fit, as long as imminent abuse is not occuring (I know that many feel JW indoctrination qualifies). The grandparent should simply attempt to reason with the child and parent without creating conflict and imposing their own beliefs on their families thus attempting to indoctrinate them in their beliefs, just as we would hope that the JW doesn't attempt to indoctrinate us.
Respect for personal beliefs is sacred. Children are completely et the mercy of the environment of their upbringing and the teachings they hear, see, and are taught. Their belief system will be formed around this. The best case scenario for any child is to be provided with a well-rounded balanced presentation of facts and viewpoints, and encouraged to decide accordingly as they grow up.
In any case a parent has the unique right and responsibility to raise their child and teach their child as they see fit. There is no getting around that fact.