With the Delta variant of Covid-19 infecting more children, many parents are worried about how to keep their unvaccinated young kids safe as schools reopen and extracurricular activities resume.

The best protection against Delta, doctors and public health officials say, is vaccination. But that doesn’t directly help children under 12, who are ineligible for the shots. So parents must weigh the risks and benefits of fall activities, from in-person school to sports, play dates and birthday parties.

Most parents by now know the basics: Masks reduce transmission and outdoors is safer than indoors. Beyond that, doctors suggest some principles to guide decision-making this fall. Give priority to your most important activities, they say, and skip others. Within your selected activities, look for ways to lessen risk.

“Almost nothing at this point is zero-risk,” says Leana Wen, an emergency physician and public health professor at George Washington University in Washington, D.C. “Do those activities and reduce risk in those activities, and then try to cut out the other activities that are higher-risk and lower-value.”

Risk accumulates with each activity, she notes. Don’t assume that if you are engaging in one higher-risk activity that you might as well do others.

Scientists are still learning about the Delta variant’s impact on children. Children, including teens, make up a small proportion of all hospitalizations, comprising roughly 1.6% to 3.5% of total Covid-19 hospitalizations among states reporting such data, according to an American Academy of Pediatrics weekly report.

However, the number of Covid-19 cases in children has steadily increased since the beginning of July, according to the AAP. There was a 5% increase in the number of child Covid-19 cases between July 29 and Aug. 12.

Pediatric hospitalizations for the coronavirus have also been rising, according to CDC data. The seven-day average of child hospitalizations reached a high of 281 for the period between Aug. 12 and Aug. 18.

For many families, in-person school will be the highest-priority activity. Public health officials generally agree that school is the most worthwhile risk for children to take. It is difficult for parents to control their child’s school environment, and some districts aren’t following public-health guidelines that recommend masking and other precautions. Even if your children’s school doesn’t require masks, you can still reduce risk by masking your own kids. Doctors suggest using a high-quality mask such as a three-ply surgical mask, an N95, a KN95 or a KF94.

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After-school care is another essential activity for many families, but different options carry different risk levels. An after-school program based in the same school that children attend may follow similar protocols, and there may be less mixing among kids, notes Elizabeth Stuart, a professor at Johns Hopkins Bloomberg School of Public Health in Baltimore.

Another reduced-risk option: Setting up an after-school group with a small number of children from the same class at someone’s house, and making sure the babysitter is vaccinated, Dr. Stuart suggests.

If a child is taking an indoor class, such as gymnastics or basketball, ask if the instructors are vaccinated. “I would want anyone interacting closely with kids to be vaccinated,” Dr. Stuart says.

As the Delta variant sweeps the globe, scientists are learning more about why new versions of the coronavirus spread faster, and what this could mean for vaccine efforts. The spike protein, which gives the virus its unmistakable shape, may hold the key. Illustration: Nick Collingwood/WSJ The Wall Street Journal Interactive Edition

Outdoors is still generally lower-risk for children to be unmasked when playing sports or on play dates, Dr. Wen says. Informal indoor social activities like play dates and birthday parties can be higher risk because they often happen without the precautions in place at schools and other organized activities.

One option is to create a pandemic pod—or revisit the one you set up last year—where families agree to socialize indoors with one or two families that share a similar level of risk. When community transmission is high, consider limiting social activities that take place in more crowded settings, such as trips to the mall or indoor movie theaters, Dr. Stuart says.

“We’re trying to find that balance of not totally locking down but also making choices that sort of move down the risk ladder,” she says.

Drorit Beckman, a parent to boys ages 7 and 9, says her risk calculus has changed over the past month. Earlier in the pandemic, she was more worried that her children might transmit Covid-19 to their great-grandmother or another adult, says the 41-year-old in Hillsdale, N.J. Now she is more worried about the risks to her children’s own health. “With the Delta variant, it’s a different ballgame,” she says. “Now it’s all about the kids. Early on it was all about the adults.”

She and her husband are still contemplating what to do about school and are talking about the possibility of home schooling. On a recent vacation they avoided indoor activities and shortened their trip by a day, skipping a planned outing to an amusement park.

“My older son loves American history and he really wanted to see the Liberty Bell” in Philadelphia, she says. “We looked at it through the window instead of going inside.”

Parents also need to try to control their own fear and anxiety about Delta, says Lucy McBride, a Washington, D.C., primary-care physician. “We have to recognize the harm that unbridled fear poses to our mental and physical health and try as best as we can as parents to model calm,” she says. “Kids are feeling our stress, too.”

And it is important to balance risk mitigation for children with the mental health risks that come from social isolation, she says. “Restricting kids from seeing their friends also has risks and does harm,” Dr. McBride says. “Their mental health matters.”

Write to Sumathi Reddy at sumathi.reddy@wsj.com