"It’s really lovely because I’ve always loved the boys and I’ve always been a long-term correspondent with their dad, it’s a very lovely feeling."
Thompson, now Dame Emma, was being recognised in the Queen's Birthday Honours for her services to drama.
She wore white trainers and a navy Stella McCartney suit to the ceremony, with the choice of designer a representation of British fashion, she said.
Sporting a Fawcett Society Equal Pay badge, Dame Emma said she hopes to use her damehood to bring attention to children going hungry in the school holidays, known as holiday hunger.
She said: "This is a very rich country with hundreds of thousands of children who don’t get enough to eat.
"I’m sorry, but that’s just rubbish. I’m going, why isn’t this top of the list? We have kids begging in schools for leftover food, that’s just awful."
The actress was joined by her long-time husband Greg Wise and their two children - Gaia, 18, and son, Tindyebwa Agaba Wise, a Rwandan orphan they informally adopted in 2003 when he was 14 - for the momentous occasion.
Watch Emma and William's hilarious interaction below.