The Princess of Wales said her children "wouldn't forgive her singing" ahead of a Christmas carol concert at Westminster Abbey.
Before the service, Catherine was heard telling singer Alfie Boe about her own musical skills, saying she needed to take some lessons as her kids probably do not think she has a good singing voice.
She said: "On the piano you can slightly hide away a bit more than you can do singing.
"Actually my children probably wouldn't forgive me - I'm not sure whether they think I've got a particularly good singing voice. I'll have to have some lessons."
When Catherine greeted her husband Prince William and two of her children, George and Charlotte, outside the Abbey, there were cheers from members of the public, with some screaming "Kate, we love you" and "Prince William, we love you".
The royal family put on a united front at the event just hours after the final part of the Sussexes' documentary dropped on Netflix.
The King, the Queen Consort and Prince of Wales joined the Princess of Wales for her festive concert on Thursday evening.
Kate, who was smiling and looked relaxed, arrived before 4.20pm dressed in an elegant deep red coat dress with matching shoes and clutch bag.
She greeted clergy at the door of the abbey before going inside where she chatted with guests and enthusiastically high-fived children.
Kate met and thanked those taking part in the service, including Paddington star Hugh Bonneville and singer Melanie C.
She also chatted with Princesses Eugenie and Beatrice before coming back out to welcome her husband and two children, who arrived before 5pm.
More than 1,800 people are gathering in the abbey for the concert, staged to recognise the "selfless efforts of individuals, families and communities across the UK, and celebrate and showcase the joy that human connection and togetherness can bring".
Kensington Palace said the second carol service Kate has held is dedicated to the late Queen and the values she demonstrated throughout her life, including "duty, empathy, faith, service, kindness, compassion and support for others".
The palace said these principles are "shared and personified by the inspirational guests who have been invited to the abbey in recognition of their tireless work to help and care for those around them".
A Christmas tree in the abbey is decorated with small Paddington Bear decorations, a nod to the late Queen's famous sketch, while guests were greeted with atmospheric snowflakes from a snow machine as they arrived at the entrance.
The palace tweeted pictures of the decorations and said: "Getting ready to welcome our #TogetherAtChristmas guests, celebrating those who have gone above and beyond in their communities and paying tribute to Her Late Majesty The Queen."
