The Duchess of Cambridge's visit to a children's cancer unit in south London left a young patient praising the royal's kindness after she promised to leave a message on his blog.

Prince William meets patient Ellis Andrews as he and his wife open a
children's cancer unit at the Royal Marsden Hospital in Sutton. Pics/AFP

Kate made the pledge to nine-year-old Fabian Bate as she opened the Royal Marsden's new �18 million
(Rs 137 crore) cancer centre with her husband William.

Caring royals: Catherine, Duchess of Cambridge meets cancer patient
Fabian Bate and promises to a personal message on his family blog

The visit was Kate's first official invite to a hospital. Following her husband's lead, she patiently chatted with the cancer patients at the Oak Centre for Children and Young People at the Royal Marsden Hospital.

The Duchess, wearing a cream Amanda Wakeley dress and LK Bennett shoes, has been making private visits to charities and good causes so she can decide which organisations she will patronise -- a process that was described as "going well" by St James's Palace.

In the centre, the royal couple met Fabian, who was in the middle of four hours of chemotherapy for his acute lymphoblastic leukaemia, and the Duchess promised to post a personal message on his family blog.

He was with his mother Lydia (50) and father Darrell, (49) as well as his two older sisters Ollie and Cassia who had come to be with him during treatment.

Bate's mother said, "William told him he is so brave, and he seemed genuinely sad about Fabian's situation. Kate asked loads of questions and said she was sorry the treatment hadn't worked first time round.

"When we told her about the blog she said 'I'll look that up and post a message'. She sounded like she really meant it so we'll see."

Fabian said, "They were kind. William said I was brave."