We've done Orlando quite a few times, December twice, October twice, August/September once, and June once.
Our favourite time to go is October. If you can swing it, try and be there for the first few days of November.
At the end of Oct 31st, they swap to Christmaz in Disney. I've never been there to see it the very next day turned into christmas- but Christmas there is flipping fab.
Universals HHN will be too scary for your kids at their ages, but Mickeys not so scary Halloween is amazing! It's a separately ticketed event, but the park is open until midnight, and they do trick of treating, a special parade, the sanderson sisters perform, it has its own firework displays, different to the usual closing show.
There are special characters out, some characters are dressed up in their Halloween costumes too, crowd times tend to be much less (they limit tickets to 25,000 opposed to the regular 100,000 generally admitted to the park)
They have special merchandise you can only get at the party, 2023 had a special Tim Burton themed popcorn bucket. I think that the candy corn sipper that DD got was a special MNSSHP merchandise item too.
I love Orlando, and Disney, but I think I'm the biggest fan of the mickeys not so scary Halloween parties!
I also find the weather to be best at that time of year, it doesn't feel quite as humid, and I think we had less rain... it does rain a lot in Orlando, but prepare for it to be warm shower temperature and prepare to keep on, keeping on! An hour later it'll all be over and the rain will have usually dried!