With other members of the security lab, I have developed crypto activities for kids that we have used at some of UW's outreach efforts (CSE's open house, GEARUP). Feel free to use these materials, but please do acknowledge me and the other original authors (and let us know).

Secret messages

A cryptanalysis activity aimed at middle schoolers, developed with Camille Cobb. Read Camille's blog post about this activity.
Materials: secret message, worksheet, and clues. Ongoing; updated occasionally.

Crypto activity packet

An activity packet to teach some fundamentals of crytography. Developed by Anna Kornfeld Simpson, with help from Camille, Ada Lerner, Eric Zeng, Alex Takakuwa, and me.
Materials: Cryptography activity packet. December 2015.