And those whose other grans are not perfect - maybe they contribute something else to their grandchildren?
One of my grandmothers taught me (without knowing it) how to let go of my children so that I would keep them, not by example but by observing from an early age the result of not letting go. I loved her as much as the "other" gran, who was much better at personal relationships, but I could see with clear childish eyes her failings, and how she could not help them.
Children are very good at seeing people as they really are, and loving them anyway.
That reminds me of a saying, that we don't love people because of their virtues, we love them despite their faults.