In case you are wondering, this is what I did with the kids.
I made the stables ahead of time using Grahm Crackers and Royal Icing. They dried at least overnight. I used scraps of cracker to prop the roof up a little, it helps with the next step.
Then I gathered the pieces for the inside: Elf Cookies, Animal Crackers, Shredded Mini-Wheats, Pretzl Stars, candy canes and gummy bears.
Use what you can find, I was lucky on the pretzel stars. I used the candy canes for Joseph's staff, and thought about wrapping Mary in a piece of blue fruit roll-up.
I helped each pair of students by squirting more royal icing on the cardboard, as I told the story of Mary and Joseph going to Bethlehem, and laying Baby Jesus in the Manger. They placed the figures in the frosting; animals in the stable, Joseph and Mary, the Manger and the Baby, and finally the star on top of the Stable.
This young man laid everyone down, but they all stand up very nicely in the icing!