Try "CHRISTMAS AROUND THE WORLD". It will show all aspects. How one particular country celebrates christmas: songs (in different languages- that would be challenging!), costumes, dances, ...etc. If you have 4 classes, each class will concentrate in one country. Then for finale, all classes will sing one song: "Christmas All Over The World"..I`ve done this last year and it was a success!
If you decide to go with that theme Australia is a great country to choose. My chn sing Aussie Jingle Bells and it talks about them being in their singlet, shorts and thongs and christmas is outside in the heat.