For a quick, but colorful, Christmas, science demonstration, have children look at the lights on a Christmas tree through a feather (a craft feather will do).
It helps if the room is dim, and they might have to close one eye, but they should see a rainbow type spectrum of color, as the light from the tree is bent and diffracted, passing through the thin slits between the filaments of the feather.
The effect is not quite as dramatic as looking at the lights through a commercially produced diffraction grating...
...but it's still pretty.
For further exploration, have children look at different types, or even different colors of lights through their feathers. Does the type of light change the type rainbow they see?
Or, do a little research together into how this effect might relate to the beautiful colors of a peacock's feathers.
Or, try out other diffraction gratings you might have sitting around the house. The Exploratorium website suggests a metal screen, two pencils held closely together, a piece of cloth, or even one strand of hair. They also have a nice (almost child friendly) explanation of what is happening to the light.