Ball Publishing has a cool new book for parents and grandparents,
"101 Kid-Friendly Plants: Fun Plants and Family Garden Projects"
by Cindy Krezel ($10).
Just in time for summer fun in the garden, Krezel's book reflects her 15-years of gardening with children. The ideas in the book are directly from her experience - not theory.
She suggests that you start with seeds because there is so much to learn from using them (I agree). Watermelon and sunflower seeds are large enough to be handled by even the smallest children.
Each of the 100 bright yellow pages pictures one plant, describes it and gives project ideas.
Other content includes: Gardening basics, Bulbs in pots, Container gardens, Plants to NOT use with children and theme gardens. Themes include butterflies, dried flower garden, edibles, five-sense garden, plant a rainbow, science experiments, etc.
The book makes it easy to imagine playing outside in the garden with children because Krezel's approach is whimsical as well as informative.
Consider planting seeds with children in your yard - they will love getting their hands dirty and watching cucumber seeds turn into supper, morning glory seeds becoming big flowers, or Lunaria money plant becoming dried flowers for the house.