We were just there this last October. We tent camped but since my gang isnt into that as much as I am I had a space heater. It worked great and all were comfortable. The park weather was beautiful. could have been 5 degrees warmer but overall I would take that over blistering heat. I would do it all over again (and will when the budget allows) I would forgo one day at the park and do the Halloween party only that day. The website implies it is an extra ticket but if you just come for the party you just need to buy for the party. They say the party starts at 7 but we got in at 4pm with that ticket.
If you try to do the pool too late in the day you will have kid-sicles. Bring sweatshirts and blankets to the sing-a-long and movie.