If vegetables are still a hard sell, try growing a pizza garden. Another idea is to plant vegetables of as many different colors as possible - red radishes, green peppers, yellow corn, orange carrots and purple cabbage make a colorful garden. Talk to your kids about the nutritional value of fruits and vegetables. Point out that different color vegetables provide different nutrients. Ask your children to read the seed packages for directions.
Some plants are easy to start from seed early and can then be transplanted when it warms up.
Gather up some small pots, recycled milk jugs with the tops cut off, or other shallow pans with a drainage hole in the bottom. Your kids can make colorful markers for each type of seed they will plant. Purchased potting soil is well worth the small expense. A starter mix with just the right formulation of fertilizer will give your seedlings an extra boost. Ask your kids to fill up the pots or dishes they've selected and they can then plant the seeds and mark them appropriately.