Place the molding clay on a solid surface and have a child shape it to look like a mountain; a mound that is wide at the bottom and narrower at the top.
Instruct the children to make a hole in the top center of the mound that is approximately the size and depth of the film container.
Have the children place the film container into the hole until the top of the container is level with the brim of the top of the volcano.
Give each child, or group of children, a teaspoon and have them scoop 2 tsp. of baking soda into the container.
Instruct the children to add about 1 tsp. of dish soap to the container, plus 1 tbsp. of yellow and red food color, if desired.
Have the children stand back from their volcano then add the vinegar into the baking soda and dish soap mixture. The volcano will "erupt" and the mixture will bubble up and run down the sides of the model.