Sergeant Lee Davidson, from Thorne, in Doncaster, was killed by a roadside bomb in Afghanistan where he was serving with the Light Dragoons in 2012.
His widow Samantha and their three children joined around 850 other youngsters whose parents died serving in the armed forces for a bash in the palace grounds hosted by Prince Harry and the Duke and Duchess of Cambridge on Saturday (May 13).
She told how the event - on the day Lee would have turned 37 - had made what would have been a sad day into a happy one for the family.
"It was a really, really nice day. The children loved it and they're still buzzing from the excitement, especially after meeting the princes and duchess" said the 30-year-old.
"Harry, William and Kate were brilliant. Harry was more of the cheeky chappy, playing with the children, but they all asked how we were getting on.
"I told them it was Lee's birthday and they all looked really sad but I said you've turned what would have been a really sad day into sort of a happy one. I call it bittersweet."
The palace grounds were turned into a playground for the day, with basketball and breakdancing among a host of activities laid on for youngsters, who also enjoyed live music from Jess Glynne and were treated to a sumptuous spread and their fill of ice cream.
The festivities finished with a flourish as the RAF Falcons put on a spectacular aerial display before parachuting into the garden.
Samantha said she and her children, Jayden, aged eight, Jamie, six, and four-year-old Amelia, who never met her father, usually mark Lee's birthday by placing flowers, cards and balloons by his grave.
Rarely does a day go by, she explained, without them talking about him and what a wonderful father and husband he was.
She said Lee would have been thrilled to see them enjoying themselves at the palace on his birthday.
"He would have been buzzing to see we'd actually met the royals. He would be stood there with a massive smile on his face watching us," she said.
"For everything the children and I have come through I think he would be really proud of us too, just like we are of him.
"The children have done fantastically well and they're very proud of their dad. I always tell them what a good man he was, and even though Amelia never met him she loves her daddy and misses him too.
"Lee was so caring and thoughtful. He always though of me and the children before himself. As long as we were happy, he was happy."
Saturday's garden party was hosted by the royal family, but Scotty's Little Soldiers - which supports children who have lost parents in action - helped coordinate the event.
Samantha paid special tribute to the charity, which she said had done a brilliant job of looking after her and so many other grieving families.