For this activity, make a pizza out of either paperboard or felt. Cut it into fourths for a kindergarten class.
If you have older kids, you could try cutting it into eighths.
Show the kids how the slices together make up the whole pizza and how you can divide it up into halves and quarters.
Let the kids role-play handing out the pizza, in accordance with how much the recipient says (i.e. "I woud like 1/4 of the pizza" or "I would like 1/2 of the pizza.").