Kate's sweet nickname for Prince Louis revealed as youngster takes on his first royal engagement

Kate has revealed her nickname for her youngest child in a sweet moment during a surprise appearance from Prince George, Princess Charlotte and Prince Louis.

The trio joined their parents, Prince William and Catherine, Princess of Wales on a visit to a Scouts hut in Slough for the Big Help Outday of charity work on the final day of the coronation weekend on Monday.

Royal fan favourite Louis delighted again with his range of facial expressions during the outing — the five-year-old's first official royal engagement.

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The family enjoyed roasting marshmallows over an open fire as a reward for their hard work, with Kate getting involved to help the kids make s'mores as Prince William held their jackets.

Prince Louis enjoyed the marshmallows off the stick, with the young royal reportedly overheard telling organisers: "I like mine just like this".

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That moment came just before his mum told him to throw the stick "in the fire Lou-bugs", revealing her sweet nickname for him.

Prince Louis, who was seen shovelling sand and dirt as well as wheeling the wheelbarrow around the property, was so engrossed with his tasks that at one point, he didn't even hear his mum ask if he needed a hand.

Clearly the youngster, who was photographed in a wheelbarrow of leaves recently for his birthday portraits, loved being out in the garden, just like his grandpa King Charles.

The youngest member of the Wales family also joined his father and brother in leaving his hand-print on a mural, again echoing another of his former birthday portraits.

The family joined volunteers in renovating the Scouts hut, with works being done including: resetting a path, installing a new drainage system, sanding and re-varnishing the front door, adding planters to the front of the building and adding a mural to create a lasting legacy of the Big Help Out's work.

Prince William, in dad mode, got in the driver's seat of an excavator and allowed Prince George, nine, to help.

On seeing his big brother inside, Prince Louis wanted a turn too, although the young royal looked a little less excited about it than his father.

Princess Charlotte, eight, got involved in putting lacquer on some woodwork, while older brother Prince George used tools to help put items together.

Prince George and Princess Charlotte undertook their first ever royal engagement last June, joining their parents in Wales during Queen Elizabeth's Platinum Jubilee celebrations.

And Prince Louis' first official outing, celebrating the King's coronation, was made even more personal for the family, with Kate the joint president of the Scouts Association.