'Prince George Is Reserved and Princess Charlotte Is Outgoing': Inside the Royal Siblings' Dynamic

Kate Middleton and Prince William are navigating a path that allows their three children — Prince George, Princess Charlotte and Prince Louis — to grow up as normally as possible while also preparing them for their roles within the royal family — roles they may already know about.

“George is more reserved, and Charlotte is more outgoing,” an insider tells PEOPLE. “Maybe it’s because he’s the heir and one day he might be King.”

George, 6, and Charlotte, 4, are already displaying personality traits that mimic their dad Prince William and their uncle Prince Harry’s dynamic.

“It’s the same difference between William and Harry,” the insider adds. “Harry is much more laid-back and William is the more serious one because he will be King one day.”

Despite George’s future responsibilities as third in line to be monarch (after grandfather Prince Charles and dad Prince William), Kate and William are treating all three of their children the same.

“There’s a responsibility that comes with that but they certainly don’t him differently,” the insider says. “They are sending them to the same school, which says everything.”

Charlotte is getting ready to join her big brother at Thomas’s Battersea in London on September 5.

“She can’t wait to be with George at big school,” says the insider. “She is so excited about it all.”

The children’s varying personalities were on display at their parents’ charity sailing race earlier this month, with Charlotte playfully sticking out her tongue at one point.

“Kate just handled it wonderfully,” says a longtime royal observer. “She just laughed, and it was really lovely to see. For once you looked at them and thought, ‘They’re a family,’ rather than ‘the royal family.’ ”

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Whether they’re stealing the show at royal weddings or casually kicking around a soccer ball while dad Prince William plays polo, the royal siblings’ relationship is strengthened by their unique upbringing — after all, most kids don’t call Queen Elizabeth “Gan Gan.”

“They are close in age, and they spend so much time together,” the source adds. “Playdates [with outsiders] can be tricky, so they learn to lean on each other.”