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.
Older children can graph the sunflower's height over time.
A younger child might enjoy tracing an outline of his or her own body on butcher paper, coloring it in, then hanging it on the wall. Help your child mark the sunflower's growth next to the model of his or her body. Make a prediction, then allow your child to refine the prediction each week. The very youngest children can practice counting in the garden. Invite your child to count the number of plants or the number of leaves on a single plant.