It really depends on how much time you have, if you are going for a week DO IT. I would never skip Epcot and our next trip we are staying at BCV and plan to be at Epcot a bit (we will have 2 preschoolers, just a month shy of 4 and 2 babies in our group.)
As many people mentioned, there is a lot of fun stuff for kids, but no "big ticket" atractions for kid. This makes for a laid back relaxed kind of day, just expolreing and seeing what you find.
Don't miss the living seas, it is great for that age and they have a new little known attraction called "turtle Talk with Crush" from Nemo. It is a "must sea" pun intened!
Garden Grill at the land is currenlty closed (CM dinner that is great.) This will be the 1st trip we don't go there.
One rather good point is if you start skipping Epcot at this age it might be harder to get them to go when the are older and have more of their own ideas. This sets them up to see that there is all kinds of fun at WDW, not just MK.
We went last year with the same group (but the kids were almost 3 instead of 4) and we felt we didn't spend enough time at Epcot.
There really is alot to do there.