They will ABSOLUTELY be usable!!! First off, the pools are heated if it's cool...secondly, many days in October are in the 80's and possibly 90's. The evenings can be cool, so I would suggest bringing a pair or 2 of long pants/jeans but BRING shorts!!! You will need them! Also, I would also suggest a fleece pullover, a sweatshirt and or a light jacket JUST IN CASE. Even on the cool nights, the buses have the AC cranked. There are laundries at the resort so if you're so inclined, you can do laundry.
Our favorite times of the year to go are March-May and September-November...it can get HOT,so be prepared!! Also, if you haven't already looked into it, I would suggest Micky's Very Special Halloween Party at night at the Magic Kingdom. You have to reserve it in advance and it is an extra charge, but you pick one evening, reserve it in advance, and you will get wrist bands for the party. The park closes early for regular guests unless you have a wristband. There is a special parade and other shows, the ENTIRE park is decorated for Halloween (I personally think it's nicer than the Christmas decorations), and I would bring costumes for the kids...they will actually be able to trick or treat in the MK!
I don't know where you're from, so I know temperatures affect people in different ways depending where you live, so keep that in mind also.
Our family LOVES Disney, the OCD on my user name stands for obsessive, compulsive Disney, so you can see where I'm coming from...LOL
One more VERY IMPORTANT piece of advise...on the first trip to the Magic Kingdom, one of the adults go in first and pick out a place on Main Street just past the train station. Get a picture(s) of the kids faces the MOMENT they see Cinderella Castle for the first time...TRUST ME, you will treasure that picture the rest of your life...I wish someone would have told us, but we have the mental memories...