As you're arriving after Easter the crowds should be getting smaller each day - but dont forget that as the crowds become less, the park hours reflect this and they close earlier.
the parks will be jam packed through April 17th--note that the Magic Kingdom is open until 11 pm nightly through April 16th, and MGM is open til 10 pm with two presentations of Fantasmic! nightly through April 17th, so the parks will all be packed. After April 17th, however, the park hours go to being pretty crummy until mid May, so the parks should be moderately calm from then onward.