A heartbreaking sentence Princess Charlotte and Prince George said to Kate made her feel " mum guilt ", the Princess of Wales revealed during an interview.
The royal is a hands-on mum and does most of the parenting with Prince William, with the help of a nanny.
But due to their royal responsibilities and duties, the couple often have to leave their three children to go to work, attending official engagements and occasionally flying overseas for tours and visits.
During an interview with Giovanna Fletcher for her podcast Happy Mum, Happy Baby, Kate spoke movingly about her life as a parent with William, opening up about details on their family life.
When asked if she feels mum guilt, the Princess of Wales replied: "Yes absolutely - and anyone who doesn't as a mother is actually lying."
She explained that being a parent is "a constant challenge" and she feels guilty when her children ask her why she does not always drop them off at school.
She said: "Even this morning, coming to the nursery visit here - George and Charlotte were like 'Mummy how could you possibly not be dropping us off as school this morning?'
"It's a constant challenge - you hear it time after time from mums, even mums who aren't necessarily working and aren't pulled in the directions of having to juggle work life and family life."
During the chat, Kate also admitted she sometimes feels guilty about the parenting decisions she makes.
She said: "[You're] always sort of questioning your own decisions and your own judgements and things like that, and I think that starts from the moment you have a baby."
And she said that to deal with the guilt she sometimes feels, she remembers something a "very wise man" once told her.
She said: "The more people you have around your children who are safe and loving and caring, the better.
"So yeah - it was a real weight off my shoulders that actually it's not totally my responsibility to do everything, because you know we all have good days, bad days - and you can dilute that with others who aren't on that particular day struggling.
"I think it makes such a difference for your child, keeping them as constant and happy as possible."
Today Charlotte is celebrating her 8th birthday and King Charles was among members of the Royal Family who sent her a special message to mark the occasion.
The official Instagram account of the Royal Family, which shares news and updates on the work of the King and Queen Consort, shared a newly-released photo of Charlotte adding a sticker that says: "Happy birthday!"
The picture was first shared late last night by the social media accounts of Prince William and Catherine, who also wanted to wish their daughter a very happy birthday.
The photo was taken by the Princess of Wales, who has a passion for photography and has previously published portrait images of her children to celebrate major milestones from birthdays to school starts.
