If you've never gardened with your child, the web sites below can help you get started.
Sunflowers are a good choice for children, because they grow quickly.
Start by planting one or more sunflower seeds. Ask your child to predict when the first seed will sprout. Once it sprouts, compare the actual day to the prediction. Your child can measure the sunflower once or twice a week. Write the flower's height on the corresponding day of a calendar.