I do this and the kids love it! There are actually several different ways to do it, but I'll show you my favorite:
1. Outside layer (crust) - graham cracker crumbled (let them crumble it)
2. Next layer (mantle) - scoop of ice cream
3. Middle layer (core) - hard candy (peppermint) They poke the candy down in it.
While the kids are eating it, I have them draw and label their earth.
I've also seen people use an apple and show it that way - with the peel (crust), inside (mantle), and core (core!).
Another way is with peanut butter as the mantle, but I usually have peanut allergies to worry about.