We always do MK first - nothing like seeing the castle for the lump in your throat "I'm at Disney" feeling. Our kids have always accepted the other parks in stride.
I sort of figure that if you little kids want to spend their time in the MK, why not let them? Aren't you there for your kids to enjoy themselves? If they are bored by Epcot, why would you spend time there? (Then again, we are DVC members, we know we will be back when the kids want to enjoy some of the grown up stuff).
We do park hop. This allows us to spend mornings in the MK, and then spend evenings, when the kids have been content to sit in a stroller, doing Mommy and Daddy stuff in Epcot. Generally, we have dinner, have one parent watch a show with the kids near the countries (Chinese Acrobats, Japanese Drummers, the band in Canada or England) while the other parent does a little browsing nearby.