The Duchess of Cambridge said children's hospices bring "joy, fun and play" to seriously-ill children.
Catherine is royal patron of East Anglia's Children's Hospices (EACH) which support those in Cambridgeshire, Essex, Norfolk and Suffolk.
The duchess was marking Children's Hospice Week with a message to the UK's 54 children's hospices.
She said they "help families make the most of their lives together, no matter how short".
Children's Hospice Week, from 20 to 26 June, is organised by the charity Together for Short Lives, which raises funds and awareness for services across the country.
The duchess said: "For any parent, coming to terms with the news your child has a life-threatening condition and may die young is heart-breaking and incredibly frightening.
"Families can often feel isolated, but having the support of a children's hospice means they don't have to face that future alone."
She said she had been "privileged to see first-hand the incredible work they do" as patron of EACH for the last 10 years.
Catherine added: "As well as providing expert care, children's hospices bring joy, fun and play to brighten each day."
She said the hospices do "extraordinary work" and provide "life-changing care and support [for] children and families going through the toughest of times".
Andy Fletcher, chief executive of Together for Short Lives, said children's hospices were "so vital" and help families "make the most of every moment, whether that's for years, months or only days".
"Hearing the news that your child will die young is heart-breaking, and families often tell us they feel isolated and alone, and just don't know where to turn to for support," he said.