I listen a fair amount to the Libera Children's Choir, and one of the things that I've noticed is that frequently they wear ear-pieces in one ear or the other (For instance, Joshua Madine in his right ear here on Youtube).
I did a bit of research on this, and didn't find anything about Libera in particular, but I did find this article on Quora that says that sometimes singers will wear them to basically block out extreanuous noises on stage. This seems rather unlikely in this case, however, since they're only accompanied by a single violin and an organ.
Why do they wear earpieces? What are they listening to?