This short PowerPoint on Volcanoes covers the different parts of a volcano (with pictures) and then finishes with a couple fun facts about volcanoes. Your children should already know the layers of the Earth before presenting this PowerPoint.
A good follow up activity is to have a cross-section of a volcano printout and then your children can label and color the different parts they've learned.
Also check out my Famous Volcanic Eruptions as a follow up.
Famous Volcanic Eruptions