When we were there in February, we did this with our 4 dd and it was great! They do cost about $10 each, but it helps make EPCOT a little more enjoyable for little ones.
One word of advice. Allow plenty of time to do this. If you start in Mexico (or on the other side in Canada) and work your way straight around, it will take you a good 4-5 hours to do it without feeling rushed. Don't make it into a "o.k., we've got this country, now let's run to the next one!" or you will miss a lot of wonderful things. They also make and decorate masks at these kidcot stations where you get the passports stamped. My girls liked that too.