Assemble a few craft items such as construction paper, crayons, children's scissors and a ruler. Make a "V" template that you either cut or simply let the children trace or draw. Use the lesson of letters as a foundation, with volcano as the example of words that start with V. Each child can make his own V. Then turn the paper or the V cutout upside down to discuss that volcanoes are mountains. Show a few images of types of volcanoes as examples.
Bring in an array of examples of volcanic rock. Pumice, obsidian and basalt chunks of different sizes can be displayed. Talk about how these rocks were once molten and then cooled and hardened. If the rocks don't have sharp edges (especially obsidian), allow children to hold them and look more closely. Magnifying glasses can be useful for closer examination. Get a large bowl of water and let the kids see what happens when you put the various stones in it -- pumice typically floats.
The smooth, ropey pahoehoe lava is very different from coarse and chunky a'a. Different volcanoes in different parts of the world may throw out ash clouds. Teach about how pressure builds up and lava is projected upward and out of the top of volcanoes. The children can add colored lines flowing to the tops of their inverted Vs to mimic this. Talk about how lava is very hot, the loud hiss it makes when meeting an ocean or body of water, and how volcanoes have contributed to the geography of the planet.
Rather than frightening young children with concepts around the destructive elements of volcanoes, focus on the positive and creative aspects. Some ideas to include are how plants grow back rapidly or how new land is added. Children may see images of volcanoes and think about "explosions" or "blowing up," so help them learn "eruption" instead, and how lava "flows." Let the children make play dough volcanoes that actually erupt safely.