Both times we've gone with kids, we've done MK every day (or almost every day). However, what we did was a little different than what you describe. We've stayed at the Poly so that it's an easy ride to and from MK, so what we've done is visit a different park in the morning, stay there until lunch, then come back to the resort for nap, and then go to MK for the afternoon for a couple hours until dinner and early bedtime. We've done a single morning at MK as well each time. This worked out really well for my young kids (first time, DD was 2.5, second time, DD was almost 4 and DS was 9 months).