The Princess of Wales said her children were "very sad" not to join her and the Prince of Wales at the Royal Variety Performance headlined by the cast of Paddington The Musical.
On Wednesday William and Kate arrived in red-carpet finery to attend their first Royal Variety Performance since Kate's recovery from cancer.
Stepping onto the red carpet at the Royal Albert Hall, William appeared in a classic tuxedo while Kate wore a green velvet gown.
The royal couple were greeted by ITV board members and representatives from the Royal Variety Charity.
Kate was presented with a posies by nine-year-old twins Emelia and Olivia Edwards, family of staff at Brinsworth House, a care home for entertainment industry workers supported by the charity.
After asking if they were fans of Paddington, the princess told the girls her children were "very sad" not to attend the show and added she had to tell them children were not allowed to go.
"My kiddies were very sad, we're going to have to keep it a big secret that I saw you guys," she said.
"They were very sad not to be joining us."
The girls then presented William with a programme for the evening's entertainment.
"Thank you very much," he said. "You two look very pretty tonight."
When Olivia told the prince her favourite singer is Billie Eilish, he replied she had good taste.
He said: "It's very nice to see you both. You're very smiley, you two."
"It's infectious," the princess added.
Alongside the cast of Paddington The Musical, pop star Jessie J and Grammy award-winning singer Laufey also featured.
The evening of entertainment also involved English band Madness and actor and Celebrity Traitors star Sir Stephen Fry.
It is the sixth time William and Kate have attended the annual charity event, and the first time since the princess's recovery from cancer.
Jessie J, who was performing her new song I'll Never Know Why, faced her own cancer surgery in June with a mastectomy for breast cancer.
The 37-year-old later revealed she was due to undergo a second operation and had to postpone her autumn tour.
Hosted by comedian Jason Manford, the evening also involved Strictly Come Dancing's Johannes Radebe showcasing an extract from the musical Kinky Boots.
Singer-songwriters Sir Bob Geldof and Midge Ure introduced a one-off performance from the Live Aid musical Just For One Day.
The grand finale was to involve 400 performers, including Michael Ball, Matt Lucas and Katy Secombe, taking part in a medley in celebration of the 40th anniversary of award-winning musical Les Miserables.
The annual Royal Variety Performance was first staged in 1912 for King George V and Queen Mary in aid of the Royal Variety Charity, which supports those working in the entertainment industry.
