Sponsored by the Cotsen Children’s Library, in collaboration with Bedtime Math, Digitopolis, the mathematical kingdom from “The Phantom Tollbooth,” will be open for visitors in the Frick Chemistry Building on the Princeton University campus from 1 to 4 p.m. on Saturday, April 9. Full of hands-on exploration, demonstrations, games, challenges, and unexpected connections, Digitopolis is for both the mathphilic and the mathphobic. Recommended for children ages 4-10. For further information, contact Dana Sheridan: danas@princeton.edu.