The Princess of Wales has an adorable nickname for her youngest child, Prince Louis. The royal mom of three was overheard calling Louis by the moniker on Monday as she told her five-year-old son to toss his marshmallow roasting stick into the fire. “Pop that in the fire, Lou bugs,” Catherine said.

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The Princess and her son roasted marshmallows during the Wales family’s engagement in Slough. As part of The Big Help Out, Prince William and Catherine volunteered with their three children at Upton Scouts Hut, where they helped renovate and improve the building on the final day of the coronation weekend.

The Big Help Out, which was a tribute to King Charles’ public service, aimed to “use volunteering to bring communities together and create a lasting volunteering legacy from the Coronation weekend.” While Louis has stepped out with his family in the past, the outing on May 8 marked his first official engagement.

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Two days before The Big Help Out, the young Prince attended his grandfather, King Charles III’s coronation. Louis was seated between his sister, Princess Charlotte, and mother at the service at Westminster Abbey. Following the coronation service on Saturday, the Wales family rode together in a carriage to Buckingham Palace, where they appeared on the balcony with Their Majesties.

Alongside footage from the historic day, the Prince and Princess of Wales’ Instagram account wrote, “What. A. Day. Thank you to everyone who made it happen #Coronation.”