Kate Middleton's nickname for Prince Louis revealed in rare public moment and we can't cope with the cuteness

Kate Middleton's nickname for Prince Louis was revealed during his first official engagement and it's just too cute!

• Kate Middleton let slip her adorable nickname for youngest son Prince Louis during his first public engagement.
• The Prince and Princess of Wales attended their first royal engagement as a family of five as they paid a visit to a Scout group in Slough. 
• This royal news comes after Queen Camilla's sweet coronation tribute to her grandchildren you probably didn't notice was revealed. 

Kate Middleton has let slip her unusual nickname for Prince Louis and its utterly adorable.

The Princess of Wales joined her husband Prince William and their three children, Prince George, nine, Princess Charlotte, eight, and Prince Louis, five, for their first royal engagement with Prince Louis in tow. 

The Wales family volunteered with the 3rd Upton Scouts Group in Slough as part of the Big Help One initiative which was set aside in tribute to King Charles' coronation. 

During the special day, they helped paint, shovel and also enjoyed eating s'mores toasted on the camp fire.

As Prince Louis began to toast marshmallows by the fire pit, Kate could be heard using the unusual nickname she has in a video shared by journalist Rebecca English, telling her youngest son, "Pop that in the fire, Lou Bug."

Fans were thrilled to see the family getting stuck in together for the rare appearance, with one writing, "These pictures are so heartwarming."

Another added, "Although Prince Louis was absent from the concert last night because it was too late for him, it was great to see him today on his first royal engagement."

One more royal fan added, "I love so much that the children are out and about learning the work of their parents."

Prince George and Princess Charlotte have a cute nickname for Prince Louis, which is Lou Lou.

Kate previously revealed that Prince George had a nickname for dad Prince William and it looks like they all go by a different name to the one they use formally.