Designer Shares Thoughts on Prince George, Princess Charlotte and Prince Louis' Coronation Style (Exclusive)

King Charles' coronation day is almost here — and the royal children will no doubt be dressed to impress.

Speaking exclusively with PEOPLE, British childrenswear designer Rachel Riley — one of whose designs Princess Charlotte wore Wednesday at the Westminster Abbey coronation rehearsal — says of the 8-year-old princess and her brothers, Prince George and Prince Louis, "Everyone is going to be dressed in a very formal way because it is their grandfather's coronation; it is such a momentous occasion."

As the oldest (and future monarch), "big boy" Prince George, 9, will likely "have a formal outfit that's been made specially for him."

And the youngest of the three, 5-year-old Prince Louis, "has always been full of character — he looks like he's a bundle of fun!"

"It's quite interesting the way each of the royal children are quite different," Riley says. "Prince George is the future king; he's the big boy, and he's at the age where he's now wearing suits. Princess Charlotte will be wearing a lovely dress, I'm sure, and Prince Louis, as the youngest, he still gets to wear short trousers, and we'll see more of the cute factor."

Of the Blossom Button Front dress Princess Charlotte wore from Riley's line to the coronation rehearsal at Westminster Abbey on Wednesday, the designer says, "She's 8, and she looks gorgeous and at ease, but she also wore the same dress when she was 4 in a different print."

Charlotte has worn Riley's Blossom Button Front Dress three times in public in different prints, while Prince George has worn nine of the designer's outfits for public appearances over the years, beginning when he was very young.

"The white shirt [Prince George] was wearing when he was presenting his new sister Princess Charlotte to the world, that same shirt, he wore for a birthday picture," she tells PEOPLE. "Then he wore it again at Christmas time when he was stirring the Christmas pudding with the Queen. It just shows the continuity. There's a lot of thought; nothing is by chance."

Riley notes that, in her opinion, "the public interest in what a real-life prince and princess wears is incredible," adding, "These images will be in the history books, and that's what's so delightful when they wear our designs."

"They love traditional clothes, and it's lovely to see them wearing things again and again," she continues of the royal siblings.

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She also stresses that her designs are meant to make children "feel good" and be "comfortable," as opposed to something stuffy and just for show.

"When [Princess Charlotte] went to the Commonwealth Games event last year, she was wearing a jersey dress of mine, her hair in plaits; she looked so vivacious and full of energy," Riley recalls. "It wasn't about the clothes — it was about her, feeling at ease."

As for Wednesday's practice for Saturday's coronation, "Princess Charlotte was thinking about her rehearsal today, not thinking about her dress, and that's my aim," she explains.

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Riley is excited to see how the "lovely event to celebrate" will all play out, noting that the crowning of King Charles, 74, is "bringing everyone together."

"It will be lovely to see all three of them at such a historic occasion," she tells PEOPLE of Prince William and Kate Middleton's children. "It is what Britain does best in terms of the pageantry involved."

Riley calls dressing the royal children "such an honor" and "one of my proudest moments" over the course of her label's 25-plus-years in business.

"It just never loses the excitement or sparkle. I'm always so delighted," she says. "To be part of so many people's family moments, it's a great treat."

And while "it's always fun to dress all children," Riley reveals to PEOPLE that "what's particularly rewarding as a designer and a business owner is that the royal children wear our clothing and they seem comfortable in them."

"I could never have dreamt — when I set up my business 25 years ago, when my kids were small and I wanted to make beautiful things for them — I could never imagine, in my wildest dreams, that one day the royals would be wearing my outfits," she adds.

"It really is an honor," Riley says.