We went a few years back and didn't do the Halloween party as my DDs were 9 and 6 at the time and I felt it was too much for them (we'd also travelled that day so were really tired!). We left the park about 7 pm I think (I can't remember what time it closed but seem to think it was early?). I would stay book Inventions for about half hour before closing, or even at closing. The ticketed Halloween party groups started arriving from about 5 pm I recall - there were lots of groups with very spooky costumes (my youngest was a bit freaked out), but they were all in great spirits - didn't appear to be any problems.
The big bonus of not doing the party which went on until the small hours, was the Studios had EMH the next morning and we got up nice and early and were one of the first there - for the first time ever we rode Crush (as it was open) 3 times in a row with no queue - was starting to build up by 3rd time. It was lovely having such a quiet Studios and made our weekend if I'm honest.
My girls are a bit older now and keep asking if we can go back for Halloween and do the party (it's my most favourite time to visit), however, sadly their school holidays don't fall over Halloween for something like the next 5 years, so we won't get on the actual day for years to come due to school! Very upset about that lol!
Enjoy - it's amazing at Halloween - love it! Hope you have some lovely costumes sorted for the children!