I have been a couple times, once before kids and about three years ago when we only had our older son (he was about 1.5 years old). As others have said, character lines are long but there are characters out that you rarely see during non-party times that are present. If your children see someone they'd like to meet, perhaps plan to wait for about 30-45 min (that was about the average for us but it could be more). At the Christmas party (since that was the last paid party event I went to/that I can remember best) we waited to see Jack Sparrow (and boy, was he spot on with facial features/voice-I swear it was like Johnny Depp in the flesh!) and that was about a half hour wait. He spent some time with us (me and my late-20 yo brother), which was nice. With kids, he was spending a few more minutes, which the kids were loving. I mention it because that could explain why you may spend some time waiting, but it was worth it for us.
One thing my little one really enjoyed? Dancing with Woody and Jessie at the Diamond Horseshoe! I'm not sure if they're doing that this year but it was super cute. William was dressed as Buzz, me as Jessie, and Hubby as Woody so it was quite the treat
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The parade is really cool, and I would take time to see that (the headless horseman was something spectacular to see-how a guy on a horse does that amazes me!).
Not sure what dates you picked but any time I've gone to one of those parties (be it Halloween or Christmas), we always picked a day Sunday-Thursday and it wasn't too crazy for us.
Remember, you can get into the park starting at 4 pm, so you could take advantage of an early dinner, some rides, shopping, etc. before the party officially starts at 7 pm-that's when all the treating, special guests, etc. come out.
Will you see everything? Maybe not, but the ambiance is fun, and the kids will enjoy wearing costumes and trick or treating in the MK. How often does that happen?!? It can be stressful for sure-getting the kids dressed up, they could be in a mood, it could be raining (happened the first time we went to MNSSHP), it's hot and humid, etc. but as long as you know that not everything will be possible, take it easy, and just go with the flow, you should have a good time
EDIT: already brought tickets for my upcoming trip in about a month too! So will be going again soon