Thursday, 12/30 should be about the best day to go to MK, simply because many local and some not-so-local kids will still be in school. The weekend will be simply atrocious for crowds and little ones. This flies in the face of conventional WDW wisdom that if you have kids you should always go to MK last of all the parks. You will definitely get good use out of your fast pass, don't be afraid to use it for the major attractions.