The particulars don't matter. The point Philip Galanes makes is, "Don't get between your grown children and their children." Our beliefs and accumulated wisdom may run counter to the parents' but unless the child is in grave danger, our role is to stay out of it.
That said, here are the particulars on what Galanes and his Social Qs suggested to a grandmother, an atheist who wanted to discuss "belief in a fictional diety" with a young grandchild whose parents have enrolled her in a parochial school.
"If you are concerned that your granddaughter's worldview is too narrow, head straight to the parents. Your daughter sounds open-minded. But you've had your turn raising children: now it's theirs. (And remember how it felt when parents and in-laws offered unsolicited advice to you?) Your granddaughter will probably appreciate hearing your philosophy when she's older. For now, wait for her to ask you about it."