And the source said William and Kate teach their children to be respectful towards anyone they might meet.
"All the children have impeccable manners and always say please and thank you.
"[The duke and duchess are] raising their kids to be well-mannered and not take their status for granted."
The proud parents recently shared a new portrait of Prince George to mark his eighth birthday on July 22. The photo was taken by keen photographer Kate, 39.
Royal writer Robert Lacey, who is also the official royal consultant for Netflix's The Crown, claims that the Duke and Duchess of Cambridge told George about his royal status "around his seventh birthday".
"William has not revealed to the world how and when he broke the big news to his son," he wrote in his book Battle of Brothers.
"Maybe one day George will tell us the story himself. But sometime around the boy's seventh birthday in the summer of 2020, it is thought that his parents went into more detail about what the little prince's life of future royal 'service and duty' would particularly involve.
"William's aim as a father, the prince stressed, was to give his son 'a normal family upbringing', enabling the monarchy 'to stay relevant and keep up with modern times'," Lacey added.
Prince William is second in line to the throne, after his father Charles. He has taken on countless charities, projects and official duties over the years in support of his grandmother the Queen.
The duke and duchess are parents to Princess Charlotte and Prince Louis as well as Prince George. In April this year, the couple celebrated their 10th wedding anniversary, releasing official photographs and a sweet video of the family. Their official residence is Kensington Palace.
William and Kate then launched their own YouTube channel in May, chronicling their activities as senior royals.
