To make a pizza garden, begin by marking out a circle. First tie a string to a stick; for a 6'-wide pizza garden, cut a string so that it's 3' long. Have one child stand in the center of the space and hold one end of the string while someone else holds the opposite end and walks around, marking the edge of the circle as he or she goes. The edge can then be marked with rocks placed around the perimeter, and the pizza garden can be divided into "slices" by the addition of more rocks or markers.
Fill the garden with vegetables and herbs that make good pizza toppings, such as tomatoes, bell peppers, onions, basil and oregano. Also ask your child to use his or her imagination to plant something that looks like cheese and pepperoni: for instance, round terracotta saucers can be used to look like pepperoni or pine straw can be scattered as mulch to look like shredded mozzarella.
Many other fun theme gardens can be used to interest children. Examples include an ABC garden, using vegetables that start with the first three letters of the alphabet: asparagus, butter beans, broccoli, cabbage and carrots are all good choices. Or you can spell out a child's name with vegetables: for "Joe" you could plant Jerusalem artichokes, okra and eggplant. Especially with young children whose imagination is unfettered, the sky's the limit.