First time we went as a family, I was determined we would do MK the first day. When I visited Florida for my first trip back in the 80s it was specifically to see Winnie the Pooh - one of my lifetime's ambitions. When I got back, I made the promise to myself that if I ever had children, I would do everything to to take them to experience the magic I did at MK and Epcot (that's all there was back then - not even Universal!).
Before we went - back in 1999 - I had read that one of the things you
shouldn't do with children is to visit MK on the first day because anything else would disappoint them afterwards
Well, it had to be MK in my book - and we did it and everyone (DH, MIL and more importantly DD, then 5 1/2) had a great time
IMHO for children a visit to MK
IS Disney and all it has to offer and all the other parks are not second best to it, they just add to the magic.
Saying that, however, now we're seasoned veterans and DD is older, we prefer to pace ourselves and (sorry all you Disney fans) prefer Universal, so we tend to to USF or Animal Kingdom.