In each pavilion in the World Showcase, there is a table set up somewhere, staffed by CMs from that country, where they have masks that kids can color. You get a blank mask from the first country you visit (they all have them) and then each station has markers. At each station, the CMs will attach a cutout "charm" to the mask, sign it, and stamp it with a passport-like stamp. This is also where you can get Epcot passports signed and stamped.
They are in a different location in each country. Some make sense, like inside the toy store in England, but sometimes you have to hunt a bit for them (or you could always ask a CM).
HTH