"Children from age 3 and up are able to differentiate between their parents' authority and their grandparents' authority," said family psychotherapist Fran Walfish, author of "The Self-Aware Parent." "A grandparent who misses a child's emotional cues and doesn't encourage expression of feeling is not nearly as impactful on the kids as a parent who does so."
You might see benefits in your children learning to navigate different styles of authority as they weave their way through a life of teachers, coaches, mentors and bosses -- all of whom will have unique personalities.
If the grandparents' behavior is harsh or borderline cruel, however, you should step in -- gingerly.
"You don't want to be in the position of telling Grandpa off in front of the child," she said. "But you want to bring the heat down and clarify for both your child and for Grandpa that there is another way."
She suggested interjecting with a gentle, "Mommy and Daddy have a different way of doing things than Grandpa does," directed at your child.
"This lets everyone know you're not interested in going to war ... but you're also not abandoning your child," Walfish said.
If a later discussion with the offending grandparent is warranted, she said, it should be initiated by the child of that grandparent -- so Grandpa's son, not daughter-in-law. It could go like this:
"'Dad, I know how much you love Johnny and care about him learning right from wrong,'" she suggested. "'But you know every one of us gets our chance to be a parent, and I don't want to miss out on my chance to do what you got to do with me. Now it's my turn.'
"If your father challenges you and doesn't accept your reasonable boundary ... it shows you that his needs come before your comfort level," she said.
That is something you should protect your kids from.