we are another family that loves epcot.
you've already gotten great suggestions on what to "plan" to do there, but i think the greatest part about epcot (ESPECIALLY world showcase) is what the little kids discover on their own...we find it to be a great adventure, and if your kids are still a bit like-minded in their interests, it will be a blast for them to wander around...point out interesting architecture (show them sculptures, let them walk up and down the steps in italy and near the moroccan fountains, etc), let them wander the rose garden path in england and find the waterfall in canada, and watch the model train outside of the germany pavillion, sample desserts from a bunch of different countries, look at the interesting things for sale in different countries (like the socks in the dept. store in japan !). walk through the tin toy museum in japan, look for lizards in the bushes, take the boat across the lake, watch for the GREAT entertainment (the "candy lady" outside the dept. store in japan, mo'rockin in morocco, and the chinese acrobats are some of our faves). if you are there for food and wine festival, there are lots of fun snacks to sample (we get 1 dessert from a number of different countries and split it between the 5 of us). if you are there for flower festival, there are ALL sorts of extra activities for the little ones (butterfly garden, crafts, garden mazes, playground equipment...it is GREAT!)
i always recommend (especially for the little ones who seem to find their own entertainment!) to WAIT on the masks until the kids need some distraction....we didn't do the masks for the first 3 or 4 trips to epcot, and now that we've done them for the last 3 trips, it seems to a bit of a distraction, i think, in that now the kids want to move from one country to another JUST for the masks...it IS great fun, and we always have a nice chat with the CMs and learn how to say "hello" and "thank you" in that language, but it takes away the "wandering" feel that encourages kids to enjoy what is around them, KWIM? as it is, we ask the kids to wait until the 2nd or 3rd day in epcot before we start on the masks.
...if nothing else, know that Sharpie sponsors these masks and all markers are permanent!!
as for WHEN to go, we like to go 1st thing in the AM, get FPs for Soarin, and get right on the Land ride, then walk over to do the Living Seas, then back for Soarin' and then go over to the "jumping fountains" in front of Imagination for a break/snack. we'll pick up FPs for something else we want to do, then head back to world showcase for lunch and do the mexico and norway ride (if the kids are up for this one).
ideally, i think you'd like to be at epcot in the morning (when lines are the shortest) AND back later in the evening (to use any of the FPs you picked up earlier in the day and for dinner)...epcot is so beautiful at night. when the kids were more interested in sitting and looking from the stroller (at 2 and 3 y.o.), DH and i would head over in the evening, and stroll WS...the kids were happy with some glow sticks and a few snacks, and DH and i had a romantic evening stroll (a margarita from mexico or a glass of wine from italy added to that "date" feel!)
i'd strongly recommend a stroller...even if as a "just in case"! *I* get tired walking around epcot, and i absolutely can't stay the whole day...i need a break in the middle!
hope this helps, and have a great time!