Sunflowers are very dramatic; they grow very quickly and get to be very big. They also have large seeds that are easily planted. Plant some sunflower seeds with your child either outside in the ground or in large pots that can be moved outside. Follow the directions on the seed packet for your area and climate. Make sure you invite your child to do the watering.
Observe the plants over time as they grow. If you and your child are ambitious, you can even make a growth chart for your sunflowers, using a very long strip of paper and a marker to record the height of the plant every week. Take a close look at the flowers once they have matured. Can you see the seeds? Keep the plants outside and watch for birds coming to eat them. Or, collect the seeds and save them in a dry place so you can plant them next year!