The Duke and Duchess of Cambridge have a few unconventional hurdles to leap when parenting their three children George, Charlotte and Louis.

First, there's the insatiable paparazzi, always keen to snap the children on their way to school.

Then there are the many confusing royal etiquette rules. After that, there are the official royal events, which weren't exactly planned with rambunctious toddlers in mind.

And then, lastly, there's the unusual custody arrangement with the Queen.

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Even the most knowledgeable fan might not be aware of this, but according to royal historians, Prince William and Kate Middleton don't have legal custody of their own children.

That, in fact, belongs to the children's 92-year-old great-grandmother, Queen Elizabeth II.

"The sovereign has legal custody of the minor grandchildren," royal expert Marlene Koenig told news.com.au.

"This goes back to King George I [who ruled in the early 1700s], and the law's never been changed. 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 is more than 300 years old and was passed by a majority of 10 out of 12 judges in 1717, who decided 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".

According to Koenig, who has written two books on the history of the British royal family and had dozens of articles published in the Eurohistory Journal, the law was legislated once again in 1772, when King George III was in power, and has never been superseded by new legislation.

The custody law still stands today, and over recent decades has affected the way the royals parent their children, especially when it comes to matters of upbringing, travel and education.

Britain's Kate, the Duchess of Cambridge smiles at Prince George as they watch Prince William take part in the Maserati Royal Charity Polo Trophy. Photo / Getty Images
Britain's Kate, the Duchess of Cambridge smiles at Prince George as they watch Prince William take part in the Maserati Royal Charity Polo Trophy. Photo / Getty Images

"When [Princes Harry and William] were little, Prince Charles asked the Queen if both children could fly on a plane together to Scotland, to which the Queen said yes," Koenig said.

"Technically, they needed permission for travel. The Queen has the last word on parenting decisions like that."

It wasn't the only time the Queen's tick of approval was sought.

An absence of the Queen's permission meant Princess Diana was not allowed to fly with her children to Australia shortly before her death, and later required Prince Charles ask for permission before he sent William, then a teenager, off on a holiday camp in the USA in the late 1990s.

Another subtle nod to the unusual custody arrangement was in the 1996 divorce agreements

between the Duke and Duchess of Wales, Prince Charles and Diana, and the Duke and Duchess of York, Prince Andrew and Sarah Ferguson.

"Custody is not included [in those divorce documents] because they did not legally have custody of their children to begin with," Koenig said.

The law also explains why Princess Diana's wishes for her brother and mother to be Harry and William's legal guardians, as laid out in her will, were swiftly ignored by the Palace. Put simply, Diana did not have the final say on the boys' upbringing or anything to do with the "care and control" of her two sons — the Queen did.

The law meant that Princess Diana's wish to have Harry and William raised by her brother and mother was ignored following her death. Photo / Getty Images
The law meant that Princess Diana's wish to have Harry and William raised by her brother and mother was ignored following her death. Photo / Getty Images

Perhaps frustratingly, William and Kate shouldn't expect to receive legal custody of their three children any time soon, either.

According to Koenig, in the event of the Queen's passing, the legal custody of George, Charlotte, Louis, and any other grandchildren in the palace will be passed onto their grandfather Prince Charles who would then be King.

The situation will be the same for any future children born to the Duke and Duchess of Sussex, Prince Harry and Meghan Markle.

Thankfully, Koenig says the palace "doesn't make a big deal" out of the law in 2018, and doubts Charles, who she describes as "very respectful of his son's parenting", would ever overshadow important or meaningful decisions.

"He understands they want to raise their children privately … the only thing Charles might ask for is more pictures," she joked.