Have your child model the layers of the earth with modeling clay or play dough. You can buy clay or play dough at your local crafts store, or you can make modeling material yourself with flour, cream of tartar, vegetable oil and salt. Choose different colors for each layer and try to reflect the difference in the depth of each layer with the thickness of the clay. After the model is complete, use a piece of dental floss or string to "cut" through the earth and reveal the layers.
Model the earth's layers with a Styrofoam ball and permanent markers. Begin by cutting the Styrofoam ball in half to reveal a cross section of the ball. Tie a pencil to a piece of string and put a tack on the other end of the string. Stick the tack in the middle of the ball and extend the string as far out as the estimated depth of that layer of earth. Tie the other end of the string to the pencil and extend the string, while going in a circle around the tack, to make a line. Continue to do this, extending the string wider and wider until you've made circles for all the layers of the earth. You can then fill in half of the cross-section with colors indicating the four different layers. On the other side of the ball, use permanent markers to indicate the names of the different layers.
An easy and edible way to demonstrate the earth's layers is with a hard-boiled egg. After the egg has cooled, use a kitchen knife to cut across the midsection horizontally. This will reveal the shell, egg white and inner yolk. If you don't plan to eat the egg, you can use a colored marker to make a smaller circle inside the yolk, indicating the inner core. You can then demonstrate that the egg shell is the earth's crust, the egg white is the mantle, the outer yolk is the outer core, and the smaller circle inside the yolk is the inner core.
Another edible way to create a model of the earth's layers is to use ice cream. Begin with a layer of vanilla ice cream flattened and lining the bowl. Then add a thicker layer of strawberry ice cream, flattening it on top of the vanilla but leaving a hole in the middle. Add some chocolate ice cream into the hole, being sure to leave a hole for the final layer, a scoop of blueberry ice cream. Then take a knife and cut across the middle of the bowl. Remove half of the ice cream and put it in another bowl. You can now see the earth's layers revealed: vanilla is the crust, strawberry is the mantle, chocolate is the outer core and blueberry is the inner core. Variations can be done with a layer cake or a pie pan with layers.