Watching something grow from a seed can be magical, and it's a great learning experience for a child. If you're already the gardening type, you probably have everything you need. But even if you're not, lots of resources are available. The best part about creating a garden for your child is that it can be as small or as big as your space and budget allow. Many stores sell gardening kits aimed at children, with small pots or seed trays, kid-sized tools and booklets explaining how to grow the plants. You could also just buy a few of your own. If you're working with a very small space, a couple of pots or a planter will suffice. If you have a lot of space, consider building one or more raised beds. They offer a lot of advantages over digging in the ground -- you can make sure the soil is high quality, and you tend to have a longer growing season, for example.
What to plant? An herb garden would be fun because herbs generally grow quickly, and your child can help you harvest and use them in cooking. Also consider other edible plants, like tomatoes or zucchini. If your kid is old enough, let her pick out some plants, too. Just make sure they're appropriate for your area and growing season. Whether you choose to grow flowers or plants for eating, start small. Be aware that you'll need to do some or most of the maintenance, like pest control and fertilizing, yourself.