Catherine, Duchess of Cambridge and Prince Louis of Cambridge

Prince Williamand Kate Middleton are mourning the death ofQueen ElizabethII with some poignant words from one of their three children.

While speaking to kids outside Windsor Castle on Saturday, Kate, 40, said that 4-year-old sonPrince Louistold her, “At least Grannie is with Great-Grandpa now,” according toThe Sunday Times' royals editor,Roya Nikkhah.

The Queen’s death follows her husband of 73 years,Prince Philip, whodied at age 99in June 2021.

Kate and William, 40 — who also share sonPrince George, 9, and 7-year-old daughterPrincess Charlotte—greeted mourners and viewed tributesto Her Majesty as they reunited withPrince HarryandMeghan Markleat Windsor in ashow of unity.

For more on Queen Elizabeth, listen below to our daily podcast PEOPLE Every Day.

After the Queendied at age 96on Thursday, making her the longest-reigning British monarch, Williamshared a heartfelt statementin his grandmother’s memory.

Chris Jackson - WPA Pool/Getty

WINDSOR, ENGLAND - SEPTEMBER 10: Catherine, Princess of Wales and Prince William, Prince of Wales speak with members of the public on the Long walk at Windsor Castle on September 10, 2022 in Windsor, England. Crowds have gathered and tributes left at the gates of Windsor Castle to Queen Elizabeth II, who died at Balmoral Castle on 8 September, 2022. (Photo by Chris Jackson - WPA Pool/Getty Images)

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“On Thursday, the world lost an extraordinary leader, whose commitment to the country, the Realms and the Commonwealth was absolute,” he wrote in part. “So much will be said in the days ahead about the meaning of her historic reign.

“I, however, have lost a grandmother. And while I will grieve her loss, I also feel incredibly grateful. I have had the benefit of The Queen’s wisdom and reassurance into my fifth decade. My wife has had twenty years of her guidance and support. My three children have got to spend holidays with her and create memories that will last their whole lives,” he continued.

RELATED VIDEO: WhyKate MiddletonDidn’t Travel to SeeQueen Elizabethin Scotland with Other Royals

As his fatherKing Charles IIIhas sinceassumed the throneand beenproclaimed monarch, William and Kate haveinherited the titles of Prince and Princess of Wales.

Their new titlesextend to their three childrenas well.

source: people.com