October should be fine in terms of weather and crowds, unless you plan to go the Mickey's Not So Scary Halloween Party. I have not been, but hear that it can get a bit crowded. My personal favorite time to go is early September right after Labor Day. First off, it is value season so the resort and the flight are cheaper. Second, most of the population is back to school or work so the parks are empty. I have walked right onto Dumbo, Big Thunder Mountain, and pretty much everything else. The lack of crowds makes touring much easier and laid back. Even though the parks tend to close earlier (usually around 8pm for MK), you get to see more things. This is hurricane season though so be prepared for rain, humidity and the possibility of something more. In the two times I have been there in Sept., I have never encountered more than a typical Florida rainstorm that lasted a few hours. Once it as very bad so I headed over to my room and relaxed with a book until it let up, then hit the pool in the evening. Although I don't have kids, I suggest this time to my friends who want to go to WDW with their kids and not miss too much. Schools usually don't start until the end of the Labor Day week or the week after, so you may be able to fit in a 5 day vacation. Plus, missing the first day or two of school can be a better option than taking her out of school one month into the term. For cooler weather and small crowds, the first couple of weeks of January are also good to WDW and may be another option. The temps can be cool though, so the pool may or may not be an option.
How long to go is a tough decision. Four days with a child is pretty minimal I would think. If you can go for 5 or 6 days, or even a week, I would say go for it. It would be less stressful for everyone involved. That being said, if you can only do 4 days, I would plan carefully. Is your daughter a very active child or does she tire easily? Would the family be able to keep up a hectic pace with a park all day, everyday or need more downtime such as sleeping in some mornings or afternoons by the pool. Without knowing, I would say get a park hopper. This will allow you to visit MK in the morning, either do a leisurely lunch or a rest at the hotel in the afternoon, then pick another park in the evening for a few rides and fireworks. Your daughter will probably be most interested in MK. There is a lot to see but in off season it shouldn't require two full days of touring. Epcot is the really huge park and can easily take 2 days, even utilizing early entrance hours, park hopping, etc. It seems to be hit or miss with kids (I clearly remember crying in the China pavillion as an 8 y.o. because I was so bored) but it is a great park and has changed quite a bit for kids in twenty years. They have kidcot stops at every country with crafts and there are more characters hanging about. Animal Kingdom is great. It is a very laid back park and may be done in half a day. I personally don't like MGM either, but it may be worth an evening visit for a few shows and Fantasmic - that's where the park hopper can come in handy!
Sorry for the lengthy post, but I hope it helps. Have fun with the planning. Just remember that there is no way to see and do it all, so just focus on having fun!
