Months into virtual school last year, Elyssa Katz witnessed her son Noah, age 9, start to lose interest in his best friend. Since the two couldn’t see each other as often, it was harder for them to keep in touch; she worried he felt lonely without his go-to friend by his side.

Once vaccinations got under way, she arranged a playdate to help reunite the two buddies who were previously “stuck at the hip.” She says it was awkward as both focused on their videogames instead of talking to each other. “They didn’t know how to play together,” says Ms. Katz, a Los Angeles consultant who matches families with tutors and caregivers. “It was pretty much two people in a room doing their own thing.”

Months later, the friends are still learning how to rekindle their friendship, she says.

Families affected by the pandemic have had to catch up on far more than math homework and spelling tests. Many parents are finding that their children have emerged from a year and half of lockdowns and start-and-stop reopenings without the much-needed benefits that come from having close friends. And as schools reopen, the adjustment back to in-person socializing could be especially difficult.

Experts say children and young adults need to share experiences such as school lunchtimes, long drives, sports practices, sleepovers, afternoon snack runs, and hangouts in basements and bedrooms for close friendships to thrive, and those things went away during the pandemic. Even teens who were able to keep up with classmates and friends via texts and social media didn’t have opportunities to create new ties or deepen old ones.

“Intimacy takes time spent together, and that’s been really hard to manage,” says Eileen Kennedy-Moore, a psychologist and author of “Growing Friendships: A Kids’ Guide to Making and Keeping Friends.” “What I’m seeing is a lot of kids who are feeling really, really sad and just kind of disconnected.”

Two hundred hours

Jeffrey Hall, an associate professor in communication studies at the University of Kansas who researches friendship, says it takes adults more than 200 hours of time spent together to become close friends and 50 hours to evolve from being acquaintances to casual friends. Although children’s friendships are less complex, he says they require similar in-person interactions or co-playing to grow stronger.

Close friendships are important in youth, experts say, because they can help children and adolescents create a road map for successful adult relationships. Children who have a best friend in elementary school feel valued and cared for in a way that affects their daily satisfaction with life and creates a “sense of self that you are a valuable person who is worth spending time with,” says Prof. Hall. For adolescents, close friendships provide a way to practice intimacy and what they want to reveal about themselves to others.

“Close friends are people who help you individuate—growing toward adult independence,” says Andrew Ledbetter, a professor at Texas Christian University who studies the impact of communication technology on close friendships.

Even before the pandemic, an increase in structured activities such as sports meant some young people had little time to nurture one-on-one friendships. Scrolling social media and comparing themselves with others just added to feelings of loneliness, says Douglas Rait, chief of Stanford University’s Couples and Family Therapy Clinic. “The pandemic highlighted the degree of disconnection that kids and adolescents have been feeling even prior to the pandemic,” says Dr. Rait.

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Some parents might be finding that the pandemic has taken a bigger toll on their children’s friendships than on their own. Younger friendships, experts say, were more vulnerable to the loss of in-person contact because they aren’t as longstanding or established as adult ones. What’s more, some parents may have felt they had to put their children’s friendships on the back burner for the sake of their family’s safety. “Parents needed to make difficult choices with the friend relationships that they were going to support and sponsor,” says Dr. Rait.

Tonia Semler in Gurnee, Ill., says she watched the relationship between her daughter, Isabella, and her best friend fizzle during the time spent apart in a virtual classroom. Getting together with her friend proved difficult throughout the pandemic. Even now that her 9-year-old is back in school, Ms. Semler hasn’t seen the friendship rebound.

So Ms. Semler encouraged her daughter to participate in soccer again to meet other friends with whom she might eventually become close. Ms. Semler feels that it’s especially critical for Isabella, an only child, to have strong bonds with her peers. Despite continuing restrictions this coming school year, she is hopeful that Isabella will be able to spend unstructured time hanging out at friends’ homes once Ms. Semler becomes more familiar with the children and parents. “I may consider a playdate with a teammate because I know them personally,” she says

Encourage social activities

Dr. Kennedy-Moore advises parents to make in-person social activities a priority for their children, much like schoolwork. They should encourage children to see friends one-on-one, and free up busy after-school schedules for casual interactions that can lead to closer connections, she says. With the spread of the Delta variant and depending on their risk tolerance, parents may want to focus these get-togethers on outdoor activities, she adds.

Experts also say that parents shouldn’t worry if a friendship from before the pandemic fizzles. In many instances, children naturally shift their close friendships as they age. Parents should also understand that the relationships children had before the pandemic may not look the same now, says Matt Lundquist, a New York-based therapist. “Think about those friendships as starting new,” he says. “Who you were 1.5 years ago might be different.”

Parents should be prepared for an adjustment period. Right now, many children are more comfortable communicating via social media than in person. So while it is critical for them to resume in-person social activities as the pandemic allows, parents need to be patient and give their children time to get relationships back on track, experts say.

For some children, the seemingly perfect friendships they see on social media may make them self-conscious about not having a best friend of their own. “I tell them, ‘You can’t compare your inside to somebody else’s outside, as you’re sitting alone in your bedroom,’” Dr. Kennedy-Moore says. It might help if parents can open up about their own challenges with keeping friendships going during the pandemic, and model the behavior they want to encourage, such as making time to socialize with friends in person, says Dr. Kennedy-Moore.

Ms. Dizik is a writer in Chicago. She can be reached at reports@wsj.com.