Place a plastic cup on a table covered with newspapers in an area you don't mind getting dirty.
Create a cone around the cup using aluminum foil. Have the cone come down from the cup at a smooth angle. The cone doesn't need to be perfect, as it is only providing a base for the modeling clay. A child can do this part by himself.
Cover the foil cone with modeling clay. Start at the bottom of the cone and work your way up to avoid collapsing the foil cone. Use a nontoxic clay if the child is doing this part. Supervise and help with the construction.
Place a funnel in the cup that forms the volcano's recess and pour the baking soda into the model volcano. Depending on the age of the child, allow her to help with this step.
Mix a few drops of red food coloring with vinegar in a separate container to give your solution the appearance of real lava. Elementary age children will have more fun if the eruption looks realistic.
Add six drops of liquid dish soap to the baking powder inside the volcano model. This will create more bubbles and a more impressive eruption.
Pour the dyed vinegar into the volcano using the funnel. The reaction will start almost instantly, so remove the funnel quickly and stand back. If the child is able, let him do this part of the experiment himself, as it is the most fun.