My daughter was age 5 on our first trip.
Intending to visit all 4 parks, I followed guidebook wisdom and went to Epcot the first day. The rationale was that once kids see MK, everything else is a bit of a letdown, so build up to that.
She loved Epcot. We loved it too - so much so that we chucked our plans and went back to Epcot the next day.
Long story short, we only made it to Epcot and MK that first trip.
To this day, Epcot is our daughter's favorite park.
Every family will be different.
That said, I would recommend 2 days at MK for sure. There are just SO many rides in that park. It's difficult to do in just one day.
I also like to recommend Epcot for a first day park. Part of it is what I said before, and part of it is that it is easier to tackle - less rides, more time to "smell the roses" so to speak.
All these years later, we like DHS ok, and just don't see much appeal to AK.
Hope this helps. My best advice is to plan, plan, plan, but always be willing to play it by ear.
