To prepare, cut a circle out of felt for the pizza crust. Cut strips of felt to represent different ingredients. I cut red circles for pepperoni, green triangles for the green peppers, etc. However, once the kids started making their pizzas, the ingredients all changed and became whatever they imagined them to be, so you can use whatever colors you have and cut them into any shape that works for you.
Let your children make their very own pizzas. I've seen the kids dump their ingredients on, arrange them one by one, and even fold them into a taco shape. Anything works!
Other ideas: Have them make a pizza just for you, and ask for a certain number of each ingredient. Or make a pizza with only a certain color ingredient. Then bring your felt pizza to the play kitchen to bake it. Use a rolling pin and pretend to roll the crust.