Another vote for the early flight.
We took an 8am flight when we last went to WDW in 2008. I had the kids clothes ready the night before with their tooth brushes & I had sippy cups for them all (but I had to dump them, well except for my youngest) for the ride.
I had breakfast for them (bagged cereal) & we bought a bottle of water in the airport for them.
They were happy & content but they woke up at a semi-normal time for them-5:50am so we could leave no later hten 6-6:15. We got to the Poly around lunch time & swam for a bit while we waited for our room to be ready.
I have taken a later am/early afternoon flight (right after lunch time) with my kids assuming they'd nap on the plane & they didn't but nicely napped on DME at about 4pm!