Boiled eggshells can be used for kid's art projects (think mosaics!) Easter egg shells work well, or just plain white and paint them with watercolor once pasted down.
If you used a non-toxic glue you could probably still compost them once you were tired of displaying the art!
Ann, Mommy to 3 beautiful, boisterous boys:
Liam - 6 1/2, Emmett - 4, almost 5
and Callan - 22 months