This will be our first time (and the kids first time at WDW). And I have the same situation as you.
We have to be at the airport before 6 am for our early morning flight (with the latest security measures/longer lines) for a 7:30-ish flight. We get to Orlando at 10:30-ish.
I know my kids will be "too excited to sleep" the night before, and I'm pretty worried about exhaustion and the late night. So we aren't doing the parks at all the day we arrive, except for
MNSSHP.
On DME we will probably get to the hotel about 11:30-ish and will go to DtD for lunch (and to buy costumes for the kids there - the whole MNSSHP is a big surprise to them - they think it is just in October because that is what the DVD said, and I didn't correct them). Then we'll go back to the resort to relax. Maybe swim a bit and then hopefully get them to nap. If I can't get them to calm down in the room, I will tell them about MNSSHP then, otherwise I will keep it a surprise as long as I can.
We'll leave the resort about 5:15-ish and take the Epcot bus, and then use the Monorail from Epcot to get to TTC then MK (their first real Disney ride - probably will do the resort monorail instead of the Express just to extend the experience). If I can keep it a secret that long, I won't tell them about the parade until we actually get to MK. They LOVE dressing up in their costumes on a regular day, so I'll just ask, "hey do you want to wear your new costumes for a bit - we can go ride the monorail train and see a highlight of what we'll get to see at the parks tomorrow".
I'm always the over-planner. I've got it all worked out. I'm SOOOO excited about surprising them, can't you tell???
I'm hoping that the afternoon nap will help. I know it will be a long day, and if they don't make it to midnight (I doubt they will), I won't fret over it.