The Duke and Duchess met heroes at Queen Elizabeth Hospital, in King's Lynn, Norfolk, on Sunday.
The royals had earlier privately marked the Clap For Carers at 5pm, a palace spokesman said.
In their first joint engagement since post-lockdown they met staff and volunteers in a marquee set up with tables and party balloons - marking 40 years of the hospital.
The party was to celebrate birthdays for both the hospital and foundation of the NHS.
They were given three small knitted key rings made by hospital staff of doctors and nurses on face masks.
The Duke said: "The children will love them."
The Duke and Duchess, both 38, spoke with hero medic Suzie Vaughan, 43, and her daughters Hettie, 7, and Bella, 9, who last month posted a viral video of reuniting with her children after two months working away from her family.
Speaking of her daughters, Suzie, an operating department practitioner, said: "They both always wanted to meet a real life princess."
The Duke and Duchess sanitised their hands before being led to meet Suzie and her daughters.
The Duke said: "I love your dresses."
The Duchess, in a blue dress, said: "You girls look really pretty."
Suzie told the royals how she kept away from her daughters for nine weeks.
Kate said: "It is a real treat to meet you both. Did you miss your mum?"
Turning to Suzie, Kate said: "You did a really good job. It's amazing how everybody pulled together."
Suzie told the Duchess her daughters had taken part in the weekly clap.
The Duke said to Suzie and chief nurse Libby McManus, "Everyone appreciated what you do. The NHS is a fantastic organisation."
The also spoke to Sam Jude, 30, a nurse at the hospital, came down with coronavirus in May, and attended the bash with his wife Blessy George, 27.
The Duke joked with health workers and asked how they coped with anxiety during the crisis.
They described "wobbly-moments".
The Duke said: "This is a good way of describing it."
Staff at King's Lynn are continuing to provide crucial support in the national response to Covid-19.
The hospital held the party on Sunday ahead of its 40th birthday on July 22nd. Dozens of frontline staff and their families had afternoon teas, sandwiches, crisps and cakes.
Households across the country returned to their doorsteps and balconies at 5pm to clap for the NHS and carers after the weekly event held during lockdown.
The NHS had called on Brits to show their appreciation before meeting together for a cup of tea.
Its website said: "Following the applause, we hope people will enjoy a drink or a cup of tea and reflect with family, friends and neighbours on the bonds that have sustained us in recent months and will continue to do so."
The hospital in King's Lynn, a short drive from the Cambridge's country home in Anmer, has provided care to 450 Covid-19 patients during the pandemic.
It has now recruited 46 patients to the Covid-19 Recovery Trial, with more than 500 enrolled in their other coronavirus research studies.
Last month the Duke visited the ambulance station at the hospital to thank paramedics.