Credit: Samir Hussein/

Prince William and Duchess Catherine reunited for a good cause on Thursday.

The pair -- married since April 29 -- opened the Oak Centre for Children and Young People at The Royal Marsden Hospital in Sutton, Surrey.

Upon their arrival, the spouses were greeted by spectators before they met with hospital staff, key donors and fundraisers. "There was excitement amongst the children before the royal arrival," a source tells Us Weekly. "One boy dressed up in an English soccer shirt to surprise William."

Because William, 29, was "on-call Wednesday night at RAF Valley and had two call outs," he didn't get a chance to sleep before the Oak Centre opening.

"He only finished at 10 a.m. Thursday and quickly got changed before he was flown over straight from the base to Surrey," a source says. See more pics of William and Kate at the Oak Centre.

When Middleton, 29, reunited with her husband at the hospital, she "gently patted him on the back" when she heard about his sleepless night. (She had been staying in London and arrived with Palace staff.)

The Duchess, who wore a pale knee-length Amanda Wakeley dress with her favorite LK Bennett nude Sledge heels and Natalie clutch, received a bouquet of flowers by brave 12-year-old Alice Marples, who has been fighting a brain tumor for eight years.

The newly opened Oak Centre is one of the largest comprehensive children's cancer centers in Europe, serving more than 5,00 patients a year.

William was appointed President of The Royal Marsden Hospital in May 2007; The position was previously held by his mother, the late Diana, Princess of Wales.