Many parents who are worried about the ramifications of so much sharing are having difficult conversations about it with their own parents, who are eager to share photos of their grandchildren with a few hundred of their closest friends.

It’s a generational tech rift involving younger parents who have been around social media long enough to understand its risks and older people who are more trusting of it. Older Americans make up a growing share of Facebook users. People aged 55 to 64 represent nearly 13% of the platform’s US users, up from 4% a decade ago, according to eMarketer.

For those parents willing to let the grandparents post at all, many try educating them about limiting posts to friends, not using hashtags and not tagging other people in their posts.

Ravelle Worthington has instructed her father not to post photos of her children that contain any location information – even their school uniforms – and not to check in to places the kids frequent, such as parks.

Ms. Worthington, a Los Angeles mom who runs Mommy Brain, an online community for new mothers, said the topic of establishing social media rules with the grandparents comes up frequently in the group.

“It’s a fine line to navigate. You’re trying to establish the groundwork for your own family, but your parents feel like, ‘We did a great job raising you, we know what we’re doing,’ ” she said.

Of course, today’s grandparents didn’t have social media, and Ms. Worthington’s father, Lester Mohammed, acknowledges it’s been a learning curve.

“The younger generation is more in tune with Facebook. I didn’t know the dangers and pitfalls,” said Mr. Mohammed.

Among parents’ concerns are that identifying data about their children—their birthdays and birthplaces – could be used in the future by bad actors. Parents also worry that old posts could one day embarrass their kids. My colleague Joanna Stern has explained why she won’t post photos of her son on social media.

Parents’ fears about their children being physically endangered by appearing on their grandparents’ Facebook feeds are arguably overblown. The vast majority of missing children are runaways, according to the National Centre for Missing and Exploited Children; less than 1% of the more than 25,000 missing-children cases the organisation assisted law enforcement with last year were abductions by strangers.

Many new parents don’t think to have the “grandsharenting” talk with their parents until after they have seen a post they don’t like, says Leah A. Plunkett, author of “Sharenthood: Why We Should Think Before We Talk About Our Kids Online.”

“If you’re leaving your infant with your mom, the odds are good that you talk about whether the baby should be given a bottle of breast milk or formula and what kind of diapers to use. But we don’t think to say to our parents or in-laws: breast milk, cloth diapers and please, no photos on Facebook,” she said.

Ms. Heller-Seid did have early conversations with her father about it – even before her son was born – because she was concerned about how often he posted photos of her sister’s children. But it didn’t do any good.

Facebook, she said, is like “a toy that he has never read the safety manual for. He has no concept of the potential implications. He just thinks it’s fun and everyone is doing it.”

It doesn’t help that her older sister doesn’t share her concerns.

“I have seen no ill effects from posting photos of my other grandchildren,” Mr. Heller said.

Despite his daughter’s preparatory talks, Mr. Heller posted several photos of her son at his bris, the Jewish circumcision ceremony, which Ms. Heller-Seid discovered shortly before her parents visited her home the next day. Although the photos weren’t graphic, she was irate.

“I have never spoken to my parents the way I did when they came through that door,” said Ms. Heller-Seid. “No one’s bris photos belong on Facebook.”

“The intensity of her feelings surprised me,” said Mr. Heller, who immediately removed the photos. “That was my comeuppance.”

After that, Ms. Heller-Seid laid down the law: Her dad wouldn’t be allowed to post any photos in which her son’s face is visible; her son has to be part of a group in any photos rather than the focus of the photo; he can’t name her child in any posts and all photos have to be approved by her before Mr. Heller can post them.

“I don’t want to dampen my dad’s joy, but at the same time I hate this,” she said.

He said he worries what his 379 Facebook friends think of him not posting photos of her child. “I feel my Facebook friends feel there’s something wrong with my fourth grandchild and that that’s why I’m not posting photos of him,” he said.

Ms. Heller-Seid, an administrative assistant in the entertainment industry in New York, shares photos of her toddler with Mr. Heller, a lawyer in Michigan, and other close relatives daily through a shared album. Mr. Heller texts her to ask permission to post photos, but her answer is usually no. He estimates he has posted no more than four photos of her son.

“I respect my daughter’s wishes. If she doesn’t want me to post my grandson’s pictures, I absolutely honour that while periodically I may beg her to allow me to post a cute one,” he said.

At a recent family gathering, Ms. Heller-Seid orchestrated a photo that included everyone in the family so that her father could post a picture. But he didn’t realize that. In anticipation of her refusal, Mr. Heller posted the photo to Facebook with her son cropped out.

Afterward, Ms. Heller-Seid told him that if only he had asked her, she would have allowed him to post the photo with her son in it.

“At that point I went, ‘Damned if you do, damned if you don’t,’ ” Mr. Heller said. “But I did turn that picture with everyone in it into a coffee mug and now I enjoy it every morning at home.”

Write to Julie Jargon at [email protected]