It was an extreme example, but it struck a nerve.

Last week, at a Walmart in Texas, a female bystander spotted a man pushing a shopping cart with a crying child’s hair wrapped around the handle. The woman intervened, and eventually called the police for help. The incident set off a debate on when bystanders should say something about another parent’s discipline .

But what happens when safety is not at stake? Is there ever a right time to tell someone you don’t agree with a parenting decision?

Gretta Keene, a Brooklyn psychologist, had a friend who was adopting a child and was insistent on never telling the child. Having dealt professionally with patients on the sensitive subject, Ms. Keene was aware of potential issues up ahead. This was a close friend, so she offered free advice: “I thought there were other ways of handling the situation and just said her choice might have unintended consequences,” she recalled.

How was this received? “She reacted by cutting off our longtime relationship.”

John Jacobs, a psychiatrist and associate professor of family therapy at N.Y.U., said, “You are always taking a gamble when discussing others’ lives, especially when it comes to their children. It’s a dangerous place to go and usually doesn’t end well.”

Two couples I know in New York reached a tense standoff when one couple’s son decided to cancel a planned spring break with the other boy. Why? Because he got a “better” offer. The boys remained friends, though the parents did not.

Similar conflicts also play out onscreen. NBC’s limited series “The Slap” dealt with the repercussions of one child’s misbehavior at a party. The incident led to cruel accusations among the adults, broken friendships and eventually, a lawsuit. The premiere of this season’s “Blue Bloods” featured the paternal paragon, played by Tom Selleck, saying to an old friend, “I would never try to tell you how to show your love to your own son, but maybe you could try seeing him for what he’s become.” The response? “Go to hell, Frank!” In the recent film, “Captain Fantastic,” Viggo Mortensen’s character is raising five kids in such an unorthodox way that the children’s grandparents fight for custody.

Indeed, the most daunting emotional tightrope involves questioning the parenting of our own children-turned-parents. With the number of “grand-boomers” growing every year, this problem is not going away. Theirs, after all, is the generation that not only thinks it will be forever young, but too often feels it is forever right. “It’s a huge issue for them to deal with their own kids’ raising of children,” said Meryle H. Gellman, a Los Angeles therapist. “Each family has to find the way because the last thing you want is disconnection.”

I recently had breakfast with three relatively new grandmothers who all acknowledged how often they hold their tongues. One feels her grandson is distant and could make eye contact more. But she is scared to mention it to her generally defensive daughter. The second asked her husband to speak with their son about his 4-year-old’s “meanspirited” behavior.

The third said she had to withhold vocal disbelief when her daughter took her 3-year-old to an occupational therapist because she wasn’t eating fruit. “I did suggest that she might be panicking too soon, and that she herself only ate grilled cheese sandwiches for one year,” she said. “When the therapist also told her that some long-term perspective was necessary, my daughter said to me, ‘O.K., but you don’t get to butt in on this a second time.’ So now we have something we call the ‘one-time only’ rule.”

Of course, there are times when some interference may be called for. Sandy (who did not want her full name used) thought her son was coasting through his first year of college until she received what she thought was an annoying call from the mother of his roommate.

“She started asking me if I’d talked to my son lately, if I had any worries and so on,” Sandy told me. “I was ready to hang up on her when she said that, apparently he had not been attending any classes and was spending most the time in the dorm room. I called the school but because of a law, they are not allowed to say anything without the student’s consent. We finally went to Michigan and found our very lost and confused son. Suffice to say, in the end, I was grateful for that phone call.”

Though every child’s life has its own trajectory, and every family its unique dynamics, there is some general advice that fits all. “Parenting is a learned skill and like any other, might need some peer or educational counseling,” said Donna Naftalis, a grandmother who worked in early childhood development. “Support groups that offer guided instruction are valuable in forming new friendships for the new parents, and can offer nonjudgmental feedback from others who have been through it.”

Dr. David Anderson, senior director of the ADHD and Behavior Disorders Center at Manhattan’s Child Mind Institute, suggested: “Share your own possible shortcomings and soften any comments by first pointing out the person’s strengths.”

My own mother was exemplary as both mom and grandmother, but it was not until her memorial service that a friend of hers told me, “Your mother said one of the great surprises of her life was how good a parent you turned out to be.” I wish I’d heard it from the original source.

In the meantime, tread gently and consider the words of the children’s book author and grandparent Judith Viorst. She wrote a book about what happened when her adult son Alexander — yes, the one who had that famously terrible, horrible, no good, very bad day — temporarily moved back into her extremely tidy home with his wife and three kids.

She notes that advice about other people’s parenting always ends up sounding like criticism, “no matter how charmingly expressed.”

“Here’s my thought on offering unsolicited advice,” Ms. Viorst said. “Don’t.”