It doesn't have to be designed or even particularly attractive, just a place where to explore without risk of damaging your prized plants. Let the children choose what they'd like to grow. Most often, these will be plants they recognize, such as pumpkins and potatoes. Plants with large seeds, such as beans, sunflowers, and nasturtiums, are easiest for small hands to sow. Though radish seeds are small, children delight in the almost instant growth and harvest.
Children love hiding places.
Consider constructing a tepee from tall poles and twine, to be covered with climbing beans and flowers. Many culinary herbs are attractive and have interesting scents. Chives, sage, mint, and basil are good choices for a child's garden. Edible flowers, such as nasturtiums, pansies, violets, and calendulas, are also good. Only a few are so toxic they should not be used around children and pets.