These kids are singing the "kalanta", a carol which brings in every home the message of Jesus' birth and says wishes. The "singers" ring triangles.
The day before Christmas hordes of children go out and knock doors, ring bells, ride trains (the photo is from metro), raid malls, even sing in the middle of the street asking for money. Of course, the custom is to be given sweets but we live in a material world :-)
Do children sing christmas carols in your country?