I’m sorry to hear that yesterday’s party was such a mixed experience.
Would you take someone to MNSSHP as their first ever time in MK? I have tickets to take my nieces (9 & 14) on the arrival evening of their first ever trip to WDW, which seemed like a great idea when I bought them. However, as I’m reading more detailed party experiences, I’m having a nagging doubt that I can’t quite identify. After the party, they will have 3 full days in WDW, 2 of which are also party nights. The night in question is late October, the week before Halloween.
Am I making a mistake in having this be their introduction to WDW and MK?
If your nieces would like to ride as much as possible without waits, then MNSSHP would probably still be a good choice. I don't know how much worse the crowds will be a week before Halloween, but when we went on Sunday, we walked right on to almost every single ride. Also, the park is so much less hot at night without the sun beating down on you. Might be a nice introduction to Disney for them. I would have really enjoyed it if I was 9 or 14. Heck, I really enjoyed it at 42. Get FastPasses for Haunted Mansion and Jungle Cruise as soon as you buy the tickets, and then get to MK by 4:00 so you can get in as quickly as possible. Jungle Cruise isn't open past 6:00, and Haunted Mansion is a great way to get them in the Halloween mood. If they want to do Haunted Mansion again later, especially to see the actors on the lawn during the wait in line, then there should be time to do it again and will probably still be a shorter wait than during a regular day.
I think they would love the trick-or-treating, too. Unless they are huge fans of some of the characters that are appearing for the party only, I wouldn't subject them to those ridiculously long waits for meet & greets. Rides and treats are the way to go.
Wishing you great weather the night you are there!