If your child can hang late, it might be worth it. If they turn into a pumpkin at 9pm, it wouldn't be. The trick or treating is lame compared to the neighborhood trick or treating they will get to do at home. It's a massive, slow moving line where you get a few pieces of candy tossed in your bag. It gets better late at night, though. Shorter lines, more candy. Still not as fun as knocking on doors in the neighborhood.
Rare characters are cool, if you don't mind long lines. Fireworks are fantastic. Parade is fantastic. Shows are fun. Lines can be short at the rides, but they are just as short (if not shorter) during the day on party days. My very favorite MK days are party days!
We did the Christmas party last year with our young 3 year old only because he can hang with the best of them. He stayed up until the very end (midnight) dancing in the snow on Main St. We tend to skip Halloween parties, now, because of the weather.