Cover your workspace with newspaper or a drop cloth. Find a board or a tray that is sturdy so that it will hold up your volcano. Set it on your workspace.
Find a narrow plastic container, such as a Crystal Light container or a narrow one-liter water bottle with the top of the bottle neck cut off. Place it on the center of the tray.
Take pieces of aluminum foil and ball them up roughly (not too tightly). Lay them around the base of the bottle. Use one large sheet of aluminum foil to cover the balls and hold them together, with the bottle in the center. This will be your armature. It doesn't have to be perfect or smooth--the bumps will add texture to the lay of the land when your volcano is complete.
Get modeling clay in various colors: black, grays and whites for stone and rocks. Brown (mixed with black or white for varying shades) for soil. Green for grass or trees. Blue for streams or waterfalls. Red, orange or yellow for lava overflowing from the crater. Refer to pictures of real volcanoes as a model and for inspiration.
Take a piece of clay about the size of a meatball and mold it in your hands to soften it and make it pliable. Working from the base and the bottom of the armature, lay pieces of flattened clay. Slightly overlap them and pinch or mold the edges to remove the seams.
Work all around the base, then work your way up the volcano. Let the clay hang over the plastic container at the top so that it goes down a couple of inches into the "crater." You may wish to start with a layer of browns and grays for dirt and rock, then go back and layer on the greenery and details, then finally a layer of lava.
Texture your clay with your fingers and any other items you have available. Use the eraser end of a pencil to create channels for streams or lava to flow. Use an old toothbrush to press into the green clay to create grass or a leafy look. Use a pencil point to gently scrape texture into your rock. You can even press real rocks, dirt, sand, small twigs or moss into the clay for a more realistic look.
Pour some white vinegar into the crater of your volcano, down into the plastic container. Fill it about halfway up. Mix it with a few drops of red food coloring. When you want your volcano to "erupt," just scoop up some baking soda into a small cup or coffee scoop and dump it down the crater, then stand back and watch it blow. Make sure the kids are standing back when you set it off.