I can't speak based on experience because I have never been during a party. We will be there the same time (9/21-9/30) and my kids are about the same age as yours (DS4 and DS22M). We have annual passes this year. So we have been going every few months. This will be our first time at Halloween. I debated about the party but decided it would be a great way to end our vacation and something very special that they haven't experienced.
My DS4 is very excited about the party and wearing his costume into the park. We've watched a few
YouTube videos so he has an idea of what to expect. I have two potential options depending on how our week goes. Either we will stay at the hotel all day. Let the kids swim in the morning, then naps, then to the party at 3:00. Or park in the morning (maybe just DS4 and me), back to hotel for naps, back at 3:00 for party. We will have had 6 days in the park so we'll just have to see.
I've heard that it will be crazy. We were there 12/29-1/6 this year and everyone said how crazy it would be. It was busy but not terrible. At least not in my opinion. There were bottle necks but in general we found it to be manageable. The party does go late but since it is a special treat we will stay as late as we can. I'm sure the little one will fall asleep in the stroller around 9. The older one may fall asleep in the stroller too. If that happens DH and I may just walk around and take in the sites.
We went to Sea World in May and my boys did enjoy it. My DS4 was actually selected to feed a shark tank which was the highlight of his vacation. My DH and I were a little disappointed because we had been to Sea World Orlando and had expected it to be on the same level which it isn't. Not sure about the park hours but when we were there it closed at 6:00 PM. That would mean you could stay until closing and then drive back to Anaheim with plenty of time to get a good night sleep before your flights. We actually stayed in San Diego that trip and drove to DL for a day since we had the AP. It will take at least 1 1/2 hours. Depending on traffic it could be a little longer. There are a few different routes.
Your flight is early but I bet based on your kids ages they will both sleep on the flight.