Help your child plan and plant this miniature garden but let them be the primary gardeners. Annual flowers are easiest to take care of and can be purchased inexpensively. Although seeds are the least expensive, it may be harder for kids to be patient. Rather use seedlings, so they can see the shoots already growing.
Plan
Let your child design and plot the shape of his/her garden.
A 4x4 foot rectangle is the easy to dig and small enough to maintain. A border garden works well, as long as your child can reach the back row. If you already have an existing garden, you could give your child a corner or raised bed of their very own. Make sure your child can reach all corners of the bed, as this makes weeding and cutting easier.
Draw out the rough shape and idea on a piece of paper with your child. Plan which flowers to plant. Is it going to be a fragrant garden, a garden for cutting flowers or a herb or vegetable garden?
Low maintenance flowers like petunia, pansy, marigold, allysum, snapdragon, daisy and cosmos are a great start. A garden book with color pictures will be a big help.
Next - a trip to the nursery.
After your plants are purchased it's time to dig, turn the soil and plant. Most kids need help with the spacing and watering the first time. Like goldfish----kids tend to over feed and water their new gardens.
After a while you will need to give a lesson on weeds. Many weeds are flowering and pretty, but will choke a new plant. Pesticides are NOT recommended for children's gardens. Ask your local nursery (or Google) for
natural pest removal tips.