I may be a voice crying in the wilderness here, but here goes:
I believe that the idea that "Mk first, other parks afterward" is very outdated.
I have brought several (Lots and lots of families) children to DW over the years. Yes, they are in love with the MK, but they also found Epcot to be a blast! I've always taken them to the MK first!!! For little ones, when we went to Epcot, we were prepared to allow them time to splash in the dancing fountains (the ones that come up from the ground) They loved it! Now, Turtletalk with Crush is a must. Loved that. The kidcot stations--couldn't get them away from making their masks at each country. For older kids, all of the interactive stuff (including the dancing instructions) at Innoventions was great. Let's see, playing with the mechanical dog, Aibo; the character bus and show, the boat ride in Mexico, as well as the boat ride in Norway, the talking water fountain in Epcot, and now Soarin'.
There's more, just can't think right now. Anyway, the children never complained that they didn't like Epcot--they understood it to be a completely different experience with Disney thrown in--and they all loved it. I made it clear to them that Disneyworld is a great big park with several gates to get into special areas, and each day we'd get to taste something new. They were just as excited to go to Epcot and MGM, and AK to experience these new things as they were to go to the Magic Kingdom. Of course we went back several times to MK, so that they got to see it all. In fact, we jumped around a lot. Never had pouty, whiney kids who wanted to go back to MK. JMHO, but I think their attitude has a lot to do with how you present it to them.