Well, our days at Epcot with kids 2, 4, and 7 would go something like this if we weren't taking a break and going back to our hotel for a midday break. Epcot is a wonderful park in which to do morning EMH if you are staying on property, so keep that in mind. Really helps you get through Soarin quicker and on to the rest of your day. Oh, and we usually have ADRs for meals planned ahead of time......preferably a late breakfast so we can take advantage of short lines at opening.
Arrive at opening, your only choice being Future World. Before you go figure out if your 3 yo is tall enough (40 inches) and brave enough for rides like Soarin and Test Track. Our 4 yo loves Soarin, and rides Test Track but always is a little apprehensive beforehand.
So, get there at opening. Grab fastpasses for Test Track and head to Soarin. If both kids can ride than all do that. If you don't think the 3 yo will go for it then do the rider swap (get the rider swap ticket from a CM when you enter the line for Soarin). One of you rides with the 5 yo while the other takes the 3 yo on the Living with the Land boat ride. Repeat that with the other parent taking the 5 yo on Soarin again, giving the 3 yo a second go on Living with the Land. If you are able to get another fastpass at this point do so for Soarin......you may want to do it again later. When you get done with Soarin you can all ride Living with the Land together if you haven't done that and are interested and there is no line. After this head over to Test Track and ride that with your fastpasses. Don't worry if the time has passed, you can still use them. Do the rider swap thing if necessary. While over there hit the single riders line at Mission: Space for whoever is interested. A definite no go for the 3 yo, and I wouldn't take my 7 yo on, much less a 5 yo. At this point it should be time for that late breakfast. We prefer the Princess Storybook Breakfast (good chance to see Mulan) over in Norway and would head over there.
After enjoying breakfast we would hit Maelstrom, since we are there anyway. Make you way over to Mexico and hit the boat ride there. From here you have some choices. You could stick with World Showcase, but we'd head back to Future World. Hit the Living Seas, see the aquarium, and do Turtle Talk. After this you can finish up whatever else you might want to see in Future World. Imagination, Honey I Blew Up the Audience, Spaceship Earth........especially if they don't have lines.
That will get you to early afternoon most likely.....and back to World Showcase. Now you can slow your pace. Start to hit the countries. Let the kids make a mask at the Kidcot Stations while you grab a snack/pastry/soft drink/beer....whatever you like.....as you soak in the atmosphere. Be sure to see the trains in Germany. Note that at this point the kids have been busy all day and you have enjoyed yourself too. Keep touring World Showcase until it is perhaps time for an early dinner at Epcot's best restaurant, Le Cellier. Order the Mushroom Filet and the Chocolate Moose.
Now you are free to just relax and take in whatever you might have missed. Were you able to get those fastpasses for Soarin earlier? Head back over there and ride again. Perhaps you didn't catch The American Adventure or the movie in Canada......give those a go. It's atmopshere time as World Showcase is great in the evening. Lots of entertainers to catch in the countries as well.
Get yourselves a good spot for Illuminations and your day is complete, a good time had by all.
At least that is what keeps our family happy and entertained all day. People who say Epcot doesn't have a lot to offer for kids don't know what they are talking about

(although I will admit a 2 yo is largely along for the ride)......plus the adults enjoy it all as well. Epcot has moved up our list of favorite parks recently.