The Prince and Princess of Wales have already "worked out" Princess Charlotte's role as a 'spare', according to a royal expert.
Prince Harry's memoir has the title Spare - in a nod to the phrase "heir and the spare", which describes him as the younger brother to future King Prince William.
Much of the book is about how Harry had to play second fiddle to his older brother - and how this affected his life.
And while promoting the book in an interview, Harry voiced concerns for the future of new spares Charlotte, seven, and four-year-old Prince Louis, the younger siblings of future King Prince George, who turns 10 later this year.
But according to royal expert and author Katie Nicholl, William and Kate's three children are enjoying a very different childhood to their father and Harry.
Speaking on Vanity Fair's Dynasty podcast, she said: "No one would want the next generation to suffer in the way that Harry has suffered. You see that suffering on the pages of his book. It's not a pretty picture for the royal family.
"So I think when you look to the Cambridges and how they are raising their children away from the spotlight in pretty ordinary schools in a pretty cottage in the grounds of Windsor, having bucket and spade holidays and teaching them to sail on the broads in Norfolk, actually they are enjoying a childhood that William and Harry just didn't get to enjoy.
"And I think that's why William and Kate are channelling everything they can into raising their children with that understanding of who they are as royals and as just ordinary children too.
"Of course, it does open up the question of the next spare, of Princess Charlotte. And I'm sure that William and Kate have that all worked out."
In an interview with The Telegraph, Harry said he worries for the fortunes of William’s children as they could “end up like him”.
He said: "I know that out of those three children, at least one will end up like me, the spare.
"And that hurts, it worries me."
Despite his concerns for his niece and nephews, he later added that brother William dismissed what he was saying.
Harry added: "He has made it very clear to me that his kids are not my responsibility."
It comes as Harry’s controversial memoir became the fastest-selling non-fiction book in the UK since records began in 1998.
According to Nielsen BookData, which collects and provides information on distribution and sales measurement of books, Harry’s headline-grabbing autobiography Spare sold 467,183 copies in its first week.
