The six-year-old son of murdered soldier Lee Rigby was one of 800 children invited to a funfair in the grounds of Buckingham Palace.
Jack was with his mum, Rebecca, who's from Halifax, at the party which was hosted by Prince Harry and the Duke and Duchess of Cambridge.
It was in honour of the children of servicemen and women killed while serving their country. Fusilier Lee Rigby was murdered outside his barracks in London four years ago by two Islamic extremists who were later jailed for life.
Prince Harry bent down to chat to Jack and Rebecca whilst trying his hand at becoming a plate-spinning children's entertainer, sharing a laugh as he struggled to keep his plate in the air.
The prince told the young guests: "We're here to celebrate you and to remind you that we as a family, we as a nation, and we up and down the country will never ever forget about the sacrifices that every single one of you have made."