Rebecca Ramsay, 42, whose 13-year-old son Dylan drowned in a quarry in 2011 has campaigning for water safety, providing talks in schools to working with the police and sector bodies to raise awareness of the risks of open water.
She attended the party on Wednesday with her mother Bernie Brown, 57, where they rubbed shoulders with the likes of the Duke and Duchess of Cambridge and Princess Beatrice.
Rebecca said: "It was such an amazing day. Dylan was with me throughout - I know he was.
"The staff were amazing. We started our day at the Waldorf Hotel in London for midday tea.
"The food was absolutely beautiful. We had apple juice and tea and Bakewell tarts, Victoria sponge and of course jam and scones.
"I bit into a sandwich thinking it was salmon and avocado but it was crab and avocado it was lovely."
"We arrived at the Palace at 3pm where there were live bands. We walked the grounds which were stunning.
"I shouted to Kate that she looked stunning and so beautiful. She shouted back thank you.
"I'm sorry I didn't get to meet the Queen. I was hoping she would have attended."
She added: "It was an amazing day and I met so many amazing people but it was always with me - the reason why I was there.
"I took a small picture of him with me. It was a beautiful day and my mum and I made memories to last a lifetime.
"I just wish I didn't get to do all these amazing things for the reasons I do. I miss my son Dylan with all my heart."
Her decade campaign has seen her awarded a British Empire Medal (BEM) for ‘services to the Prevention of Water Related Accidents’ alongside Petition Campaign of the Year.
How will she be celebrating the Queen's Platinum Jubilee?
"The Jubilee I will without a doubt be doing something. I've been invited to attend Blackburn cathedral.
"I'm still fighting to have water safety taught on the national curriculum and things are changing but not quick enough. It needs to be mandatory and or Ofsted regulated."