M.L.
You can have an adult narrate the Christmas story, and the children can sing songs between short passages. Since you only have 15 minutes, the speaker can use a few short versus, or just bear testimony in between songs about different meanings of Christmas and Christ. Each speakers words will set up for the upcoming song the children sing. Even cuter, would be if the speakers were young children, just telling a sentence or two
Here's an example, it's more adult oriented, but words and songs would be changed for the time and the kids singing.
http://www.mormonshare.com/lds-activity/jennys-super-simp...
Here is a nativity pageant script for young children:
http://www.jennysmith.net/lds-clipart/nativity-pageant-sc...