When a child becomes a parent, roles shift.

In the days after her first grandchild was born, Randi Heredia listened to her son, a new dad, ask her to wash her hands before picking up the baby and warned her not to kiss the baby’s face.

It caught her off guard. “I’m so used to telling my son what to do and what not to do,” she says. Still, watching him step up as provider and protector was affirming and liberating. She doesn’t have to be the one making decisions and setting boundaries.

Sons and daughters will always be your child, but that identity and role becomes secondary when they have a baby. You have to let go of being the boss, which can be hard for parents used to jumping in and fixing things, says Gertrude Lyons , author, educator and parenting coach .

The best route is to let them be the parents, respect their boundaries and be grateful, because not everyone has the good fortune of seeing their kids have kids. The U.S. birthrate is falling and more young adults are deciding not to have kids. Of American parents between the ages of 50 and 90, some 35% don’t have grandkids. In 2018, the share was 30%.

Aaron Larsen ​ says many grandparents still hold a traditional view that they are entitled to spoil their grandchild, whether that means giving them ice cream before dinner or overloading them with toys, rather than being entrusted with their care.

“That doesn’t jibe well with this generation of parents,” says Larsen, a father of two young children and the founder of GrandparentsAcademy.com, which offers online classes and conferences.

Advice no-nos

Younger parents search online for information on pacifiers, first cereals and sleep schedules. They might ask their own parents for advice. But they will weigh it against what they have researched. Often, the internet wins.

Taylor Wolfe , who has two daughters, 2 and 4, says millennial parents have too much information available about every possible hazard and disease. And too many devices.

She had an expensive crib with motions and sounds to calm a baby, but it didn’t work for her first child and is now in the garage. Wolfe, an author and comedian, and her mom began making TikTok videos together about millennial moms leaving kids with boomer grandparents. They are lighthearted but hit common tensions.

Her mom doesn’t understand why her 2-year-old still sleeps in a sleep sack. Tantrums are another issue. Her mom’s approach is “Knock it off.” Taylor is more about acknowledging emotions, but will resort to “Knock it off” if that doesn’t work.

Viewers responded. One woman commented that she recently found a Word document with instructions for babysitting one child that she put together for her in-laws eight years ago. “omg I was INSANE,” she wrote. “By the 2nd/3rd baby I was like ‘just keep them alive, bye!’”

Parents look at detailed instructions about nap time, snack time, bath time, and bedtime and say, or think, “Wait, what? I’ve done this before.”

Their kids acknowledge this. They also point out that their parents put blankets and bumpers in the crib—a no-no today.

“Even though you may have more experience, your kids may not want your advice,” says Joshua Coleman , author, clinical psychologist and senior fellow at the Council on Contemporary Families. “They may want to figure it out themselves.”

It’s a delicate balance .

The grandparents’ kids are adults and need to step up as parents, but they might need advice. Knowing how to offer it can help.

Heredia, the new grandmother, asked her son to think about adding a little background noise while the baby sleeps, rather than trying to maintain silence. She said it had helped her as a new, young mom.

She was more comfortable making the suggestion to her son, rather than his fiancée. “We have a great relationship, but I’m not her mom,” she says.

The experience has made her think about her own mom, who offered help and suggestions when Heredia had her children. At the time, she politely declined.

“I got this. I know what I’m doing,” Heredia recalls telling her mom. “Looking at things from a grandparent perspective, I feel kind of bad. She just wanted to offer advice.”

Puppy boundaries

Some advice is welcome. Some isn’t.

Becki Cohn-Vargas was thrilled when her daughter had a baby boy in 2020 but less thrilled when her daughter bought a large puppy. Cohn-Vargas offered to take care of her grandson and the puppy at her house, but the playful puppy scared her own older dog.

“It was not working out,” says Cohn-Vargas. She told her daughter, Melania Khouie-Vargas , that she thought it was a mistake to get a big dog and suggested finding another owner or giving the dog back to the breeder.

Her daughter said she wasn’t getting rid of the dog but agreed to send the puppy to doggy daycare when her mom provided child care.

“We moved on and it was fine,” says Khouie-Vargas. She often seeks her mom’s advice and doesn’t set strict rules about what her parents can and can’t do with her son.

She wants them to have a relationship with her son independent of her. Having had her own child, she admires her mom—who raised three children and was a school principal, superintendent and author—even more. “I’m in awe of how she did it,” she says.

Admiration often works both ways. David Buck and his wife, Susan Rose , freely admit their son and daughter in-law are better equipped for parenthood and more structured in their approach than they were. Susan relied on one book, What to Expect When You’re Expecting, when she was pregnant. Her boys were about 10 pounds at birth. She remembers her parents pushing her to feed them rice cereal.

“There was a lot of free advice, whether we wanted it or not,” says David. “We’ll provide it, if asked.”

Write to Clare Ansberry at clare.ansberry@wsj.com