I've just had my first 2 turn up, older sister with her little brother. They sang the first bit of We Wish you a Merry Christmas, and then there was a brief pause - I was all ready to ask where the rest of the song was - but they continued with it.
The older girl then sang Jingle Bells (longer version) on her own.
Realised I don't really have anything to give Carol singers........and not really sure what you give them anyhow