Princess Catherine told her kids about her cancer. Here’s how others can.

“It has taken us time to explain everything to George, Charlotte and Louis in a way that is appropriate for them and to reassure them that I will be okay,” said Catherine, 42.

The Washington Post asked professionals about how to talk to children about a cancer diagnosis. Here’s what they said.

When should parents tell a child when they have cancer?

The earlier parents tell children about their diagnoses, the better, experts told The Post.

Most children can intuit when something is going on in their family, according to Abbie Owens, who specializes in psychosocial and emotional needs of children treated at the Huntsman Cancer Institute at the University of Utah.

“It can be hard initially to even say the word ‘cancer,’ but it forms a really good foundation of trust that you’re all in the journey together going forward,” Owens said.

Depending on the age of the child, she said, parents should use language that they can understand: “Mom’s cells are growing too fast, and she has to take strong medicine to try to get better.”

How can parents tailor the discussion depending on children’s ages?

Children may receive the news differently, but parents generally know best how to deliver it. The most important part is to normalize and validate their feelings.

For children under age 5, it’s good to use a simple sentence to teach them about cancer.

“Mom or Dad’s body has something called cancer,” Owens said as an example, suggesting parents include the idea that there will be many doctor visits.

Children this age do well with play, so getting them a toy doctor kit with a stethoscope, a thermometer, bandages and a doll could help them understand what’s going on and where in the parent’s body.

Parenting coach Chelsey Hauge-Zavaleta says picture books about cancer can help young children understand what the disease is and does. She says it’s okay to give them a little bit of information at a time so they have space to ask questions and process the news.

Children ages 5 to 11 can usually understand more specifics about the side effects of treatments. Parents should also give kids ways to participate, Hauge-Zavaleta said. She says she’s honest with her three children about the drugs she takes to keep her breast cancer at bay.

“I might say to an elementary-schooler, ‘After Mommy gets her medicine, I might have to sleep for two or three days. While Mommy is resting, you get to help Daddy go make the dinner or you can draw a picture for Mommy so she can put it on her desk,’ ” she said.

Older children may know more about cancer and be more afraid.

Jessica Reid Sliwerski, author of the children’s book “Cancer Hates Kisses,” suggests speaking to an older child before younger ones. Older children tend to have more questions, and parents should be ready to “help allay those fears and those concerns, while also being very transparent and realistic that you’ll have a lot of doctor appointments and the treatment may be intense,” she said.

Reid Sliwerski was diagnosed with breast cancer four months after giving birth, and she says wrote the book in anticipation of questions her children would have.

How can parents help children cope with a cancer diagnosis?

Owens, of the cancer institute in Utah, said some children are resilient and don’t need additional support. However, if they show behavioral problems, or if their development regresses, that’s a good time to bring in a therapist or another professional.

Reid Sliwerski said parents should ask children whether they would like someone to talk to about any issues. She also says family therapy could be good support tool.

Hauge-Zavaleta, the parenting coach, said parents should increase support networks as much as possible. This could mean seeking out support groups or specialized camps for children with parents diagnosed with cancer, such as Camp Kesem or Pickles Group.

Owens advises parents to create a designated space where children can use toys or tools to explore their feelings.

“Maybe you have, like, a little basket in a corner of a playroom or something that has, like, some Play-Doh, stress balls, coloring books, or notebooks to journal or draw,” Owens said. “These are good clues to also find out how your child is feeling.”

Owens said speaking to children about cancer doesn’t need to be daunting.

“If you don’t have the energy, it can be just like a loving, honest conversation,” she said. “Be gentle with yourself. Going through cancer and being a parent is very unfair. Remind yourself to have grace for yourself.”