Harry said he wishes Archie and Lilibet could have met his late mother who he said “most certainly will never be forgotten”.
The duke said: “My dear friend Prince Seeiso and I founded Sentebale 16 years ago in honour of our mothers, with a mission to support many of the most vulnerable young people on the planet – and to give them care and help them thrive.
“In Sesotho, the language of the Basotho – the word Sentebale means ‘forget-me-not’.
“Next week is the 25th anniversary of my mother’s death, and she most certainly will never be forgotten.
“I want it to be a day filled with memories of her incredible work and love for the way she did it.
“I want it to be a day to share the spirit of my mum with my family, with my children, who I wish could have met her.
“Every day, I hope to do her proud. She was tireless in her work to support and destigmatise those experiencing HIV/AIDS. Fittingly, her favourite flowers were forget-me-nots.
“I hope we can remember my mother’s legacy by recommitting to those we serve, whoever and wherever that may be.
“For Sentebale, our commitment is to ending HIV and providing meaningful care and support for the children and young people whose lives have been affected by this disease. Because they too will never be forgotten.”
Diana died in a car crash in Paris on August 31 1997.
Her legacy lives on in sons Harry and William through their work championing causes close to her heart.
Harry has taken on the cause of supporting those with HIV/Aids through Sentebale, and by publicly having tests for the virus – including with pop superstar Rihanna in 2016.
Earlier on Thursday, Harry rode to victory in an annual charity polo tournament in Aspen with commentators praising his “superstar” play in the Sentebale ISPS Handa Polo Cup.
During his speech after the polo, Harry said: “At Sentebale we strive to break generational cycles of trauma and stigma every day.
“By educating and empowering young people to know their status and to know they can live long, healthy lives while on medication, we aim to end HIV once and for all.
“By being here today, you are a part of our journey. So, we thank you. I thank you. And if she were here, I know my mother would thank you too.”