It is very important for grandparents to tell parents about their feelings in this matter. This should be done in a loving and uncritical way with emphasis on the profound importance grandparents place on transmitting their religious legacy. Then it s up to the parents. The best many grandparents hope for is the ability to expose the grandchild to religion as a didactic, rather than proselytizing exercise. Put this way parents seem to more easily agree.
Equity is also important.
When sets of grandparents are of different religions equal time should be allotted to learning about each religion. Here are some guidelines for grandparents to follow. Have a family conference with parents to hammer out a family policy so what you do with your grandchild has the support of the parents. Explore many belief systems. View exposure to different belief systems as education Teach, don't sell your religion to your grandchild.