For many parents coming to terms with a cancer diagnosis, it is the hardest conversation to have. How do you tell your children that you have a serious illness, and that you can’t be certain of what the future holds?

In her video statement released on Friday, the Princess of Wales said the news of her cancer had been a “huge shock”. It had taken her and William “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 OK”.

It is assumed that Kate and William will spend the Easter holidays helping the children to understand and accept their mother’s illness. In their case, a family crisis is also a global news story, and great effort will go into shielding the children from relentless media coverage.

At 10, eight and five, George, Charlotte and Louis will have different levels of understanding. But the consensus from cancer charities and oncologists is that there should be as much transparency as possible.

It wasn’t always the case. In the past, cancer was a word spoken in hushed tones and never mentioned in front of children. Some experienced the anguish of a parental death without knowing the cause for many years.

Now, according to Cancer Research, “being open and honest is almost always the best way with children. Remember that uncertainty or not knowing may be harder for them to cope with than the truth”.

The charity advises parents to tailor the message to the child’s age. Younger children will only have a basic understanding of illness, and may worry they have done something to cause the cancer or that it could be contagious.

They may not be able to articulate how they feel, and instead express themselves through mood or behaviour. Younger children may be more clingy than usual and regress back to old behaviours such as thumb sucking, tantrums or bed wetting.

They might seem withdrawn, or develop new worries and fears such as being scared of the dark or dogs. Some may develop physical symptoms such as headaches or stomach pains, or may have trouble sleeping or have nightmares. Others may show no reaction at all.

Older children have a “better understanding of how the body works”, the charity says. They can understand that people can have serious illnesses. They also may instinctively know if information is being withheld.

Macmillan Cancer Support says: “It is best to be honest with children. If they think you are being vague or hiding something, they may find it hard to believe they are being told the truth. Do not make things sound less serious than they are.

“It is fine to say you don’t know if you cannot answer all their questions. Tell them you will try to find out and will tell them when you know.”

The timing of a conversation with children about cancer can be difficult. Some people tell their children as soon as they are diagnosed. Others choose to wait until test results are back and more is known about the treatment plan.

“There isn’t always a right time. But keeping it secret can be stressful. It is likely that your children are aware something is wrong,” says Cancer Research.

If children ask directly if a parent is going to die, “it is important not to dismiss this question as most children will be thinking it, even if they don’t say it … It is important that you avoid lying to your children.”

Laura decided not to tell her children anything when she was first diagnosed with cancer. She had just given birth to her third baby; her older children were four and eight. “She told them nothing, they were completely oblivious, and everyone else in the family was silenced,” said Jennifer, her sister.

Five years after the diagnosis, Laura was declared clear of cancer. But a year later it was back, and it had spread. “She talked to the older children, but she said she’d be OK. She played it right down,” said Jennifer. “She didn’t really accept it herself.”

Laura died 18 months later, in 2011. “It was a shock for the children. Her eldest said she’d promised them she wouldn’t die. They were totally unprepared.”

Last year Jennifer was asked by a friend, Claire, to be present when she told her children, aged 10 and 14, that she had just been diagnosed with cancer. “The younger one looked a bit startled and confused, and the teenager was texting her friends as her mum was talking – it was her way of dealing with it.” Neither child asked any questions.

Seven weeks later, Claire died at the age of 51. “She told the children that she wouldn’t see them growing up, but it was not her choice, and that she loved them. It was the conversation my sister should have had,” said Jennifer.