A nine-year-old from Devon has helped Prince William open a new Help for Heroes rehabilitation centre at Headley Court in Surrey.
The new facilities at the armed forces treatment centre feature a range of resources including a new swimming pool and gym.
Lydia Cross is a child patron of Help for Heroes after losing both her legs to meningitis.
"It felt really exciting, but I was really nervous", she told BBC News.
"I think I can make a difference about what happens to these injured lads, so I think I should help them," she added.
Opening the centre Prince William paid tribute to staff and the charity: "What is achieved here is, in truth, but the tip of the iceberg.
"This country has been elevated by Help for Heroes to a state of realisation and proactive support for our military that has made me very, very proud to be British."
Lydia's mother Jodie Cross described the day as "overwhelming" for her daughter but said meeting other people like her had been "really good for her".
She said her daughter supported the charity after realising it helped people who, like her, have lost limbs.
Prince William and his brother Harry have previously met patients undergoing treatment at Headley Court after being injured in conflicts in Afghanistan and Iraq.
The centre began its work after World War II for RAF personnel, but in 1996 became the main UK military rehabilitation centre for all three armed forces.