The Duchess of Gloucester was at Southwark Cathedral for the Children's Trust's 25th anniversary service on Thursday morning.
The charity was formed in 1984 to save Tadworth Court Children's Hospital which had been founded as Great Ormond Street's country hospital.
Her Royal Highness was welcomed in the churchyard by the Dean, Colin Slee, and Mayor of Southwark Jeff Hook who was undertaking the first engagement of his mayoral year.
At the opening of the service television actress Amanda Burton, Patron of the Trust, read The Special Child.
A testimonial was delivered by Alfie Russell who was 10 when he was knocked off his bike by a car.
"My Mum and Dad were told that I would never walk, talk or even eat again," said Alfie. "They were warned I could be blind and that I would be 'vegetable'." He said that he arrived at the hospital on a stretcher and left on a new bike.
He added that when the nurses from Guy's came to visit him at Tadworth they couldn't believe what he had achieved.
"Since then I have gained a gold medal in the Special Olympics and in 2008 I ran in the London Marathon to raise money for the trust."
The service concluded with chief executive Andrew Ross leading the renewal of the trust's commitment "to address the needs of children with disabilities across the country".
Elaine Paige was one of many high-profile supporters of the charity in the congregation.
Earlier in the day an anthem was sung from the cathedral tower by Guy's medical students to mark Ascension Day.