An egg-scellent craft idea from good ol' Martha
Young gardeners can use eggshells as pots to start seeds and coffee-stirrer tags to foretell what will pop up where. Plant seeds according to package instructions, and nestle planters in an egg carton on a sunny windowsill, where they can be watered easily. The first leaves to sprout will be the cotyledons or seed leaves, which supply nutrients to the young plant until the first true leaves (resembling those of the parent plant) appear. When plants have grown to about 3 inches and have at least two sets of true leaves, they are ready to be transplanted to the garden. (Remove them from the eggshells first.)
Read more at Marthastewart.com: Eggshell Flowerpots - Martha Stewart Crafts
Seeds that are larger and quick to germinate are best for kids. These include tomatoes, zinnias, and marigolds to name a few.