According to royal expert Marlene Koenig, even second-in-line to the throne Prince William and wife Kate Middleton don't legally have full custody of their three children - Prince George, Princess Charlotte and Prince Louis.
This is because the family has an agreement with the Queen, which means she is granted full legal custody over all minor royals.
Koenig further explained the rule to the Sun Online. She said: "The sovereign has legal custody of the minor grandchildren.
"Legislation passed during the reign of George I. It was known as The Grand Opinion for the Prerogative Concerning the Royal Family and it was about the King’s control over the education, the raising and the marriage of his grandchildren.
"He did it because he had a very poor relationship with his son, the future King George II, so they had this law passed that meant the King was the guardian of his grandchildren."
The law dates back more than 300 years to 1717, a time when the monarch's "right of supervision extended to his grandchildren and this right of right belongs to His Majesty, King of the Realm, even during their father’s lifetime".
The Queen also had full custody of William and Harry when they were younger.
This fact first came to light when Prince Charles and Princess Diana got divorced in 1996.
In the couple's divorce agreement there was no mention of custody arrangements for the children, despite arrangements being made.
This was because neither parent had full custody, instead this belonged to the Queen.
This meant Charles and Diana had to get the Queen's permission before doing simple things with their children, such as going on holiday.