Crowds turned out to celebrate Queen Elizabeth‘s 93rd birthday, but at the June 8 Trooping the Colour parade all eyes were on Prince William, his wife, Kate Middleton and their three adorable children, Prince GeorgePrincess Charlotte and Prince Louis — although, a fourth baby may be added in the near future.

George, 5, watched with awe as jets flew overhead while his little sister, 4, expertly waved from the palace balcony and little Louis, 14 months, clapped his hands in excitement. “Kate knew Louis would love watching the planes,” a friend exclusively tells Closer Weekly in the magazine’s latest issue, on newsstands now. The royal pair — looking gorgeous in their formal wear — smiled broadly as they attended to their kids with love and patience. “They adore being parents,” says the pals, who confides that the two aren’t done growing their family.

Prince William Kate Middleton
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“William and Kate definitely want to have one more child — they’d love to have two girls and two boys!” the friend reveals.

The Duchess of Cambridge, 36, who grew up close to her younger sister, Pippa Middleton, dreams of watching her only daughter bond with a new baby sister the same way that George did with Louis. “He always boasts to his friends about being a big brother,” the pal says, who adds that the royal couple have a gift for raising happy children — even though they are all quite different.

Princess Charlotte
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“Charlotte is the most confident,” says the friend. “George is more reserved, and Louis is in the middle. He loves listening to music — he puts William and Kate in hysterics when he tries to dance!” the friend gushes.

The timing for adding a new baby also couldn’t be better. In September, Charlotte will begin attending the same full-time school as George, leaving just Louis at home all day. “Will and Kate’s kids are their world,” says the friend. “They at their happiest when they are surrounded by their family.”

Prince William
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At the moment, the royals have their hands full with Meghan Markle and Prince Harry‘s first child, baby Archie, but maybe the family will be busy with another child soon enough!

For more on the royals, pick up the latest issue of Closer Weekly, on newsstands now — and be sure to sign up for our newsletter for more exclusive news!