If you want it to remain soft, the kids could enclose this layer in a zip-lock bag and squeeze out the air before sealing it. Use a different colored clay or dough to make this layer. Again, it could be sealed in plastic so it will remain soft. Cut a thin piece of cardboard to serve as the boundary between the crust and upper mantle. On the bottom side of it spread a thin layer of clay or dough for the upper mantle and on the top side spread a thin layer for the crust.
Students can create mountain ranges in the crust.
The kids could tape a nail and nickel on the box near the core to show that it is made of iron and nickel. With the information already provided, students could write a few words on the outside of the box about the characteristics and dimensions of the layers. Two batches of the recipe will make enough dough for five models. 4 c. baking soda 2 c. cornstarch 2.5 c. cold water Mix all ingredients in a saucepan and cook about 10 minutes over medium heat until the consistency of mashed potatoes. Remove from heat, empty onto a plate, and cover with a damp cloth.