Kate Middleton’s kids, Prince George, Princess Charlotte and Prince Louis, are a special source of support as she receives cancer treatment. 

"You take heart from the extraordinary resilience of children," a longtime family friend tells PEOPLE exclusively in this week's cover story.

The Prince and Princess of Wales are getting through her health crisis with support from Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5, who "buoy them with smiles back," the family friend says.

"Having fun together when they can is very important," the insider adds.

"The children are at the center of their world," the friend adds about Prince William, 41, and Princess Kate, 42, which was made clear in the royal's cancer announcement and the timing of it. The Princess of Wales waited to make the public announcement about her health until after her children were out of school for the Easter holiday. George, Charlotte and Louis aren't due back in the classroom until later in April, creating space for the family to spend private time together.

"We now understand the logic of the timing — waiting for the children to break up from school. And she struck a pitch-perfect note," royal biographer Sally Bedell Smith tells PEOPLE about Princess Kate’s video message, which sent shockwaves around the world on March 22.

The Prince and Princess of Wales' family subsequently skipped the royal family’s church outing on Easter, breaking tradition from past years.

The Princess of Wales spoke from the heart in the self-written address filmed in the garden of Windsor Castle, where she explained that she was undergoing "preventative chemotherapy" after tests following her abdominal surgery in January "found cancer had been present." Kate said that she and William took time to "explain everything to George, Charlotte and Louis in a way that is appropriate for them and to reassure them that I am going to be okay."

"As I have said to them: I am well and getting stronger every day by focusing on the things that will help me heal; in my mind, body and spirits," the royal mom said about how she is talking to her kids about her cancer treatment.

As the Princess of Wales leans on Prince William during this time, she has a real partner in her husband. 

"He is a very reliable, strong person. She doesn’t feel isolated in this because of William, who is right beside her," a friend tells PEOPLE. 

"This is about supporting her and her feeling supported. It is a joint effort. She doesn’t feel isolated at all," the friend says. "The nuclear family that they have created so well is an immense support not only to him but to Catherine too."