Don't worry, you'll be fine!!
As to clothes, think LAYERS!!! DW in Jan can get cold in the evenings. Last Jan we went for 7 days (DH, me, DD 7, DS 6 & DD 5) and I packed 2 pairs of long pants for everyone in the family and sweats for the plane ride. Turns out, we wore them every day, but with a washer and dryer, no problem! I think we only broke out the shorts one day, but be prepared with them also! And for nights we the temps dipped into the 40's, we wore long sleeve t-shirts with sweatshirts and windbreakers over them and were fine. When we watched Fantasmic, we bought beach towels as souveniers and used them like blankets for extra warmth!
I think last Jan was colder than normal, so hopefully you'll get warmer days than us! My other planning tip is BACKPACKS! Use them to store kid activities for the plane/car ride. Empty them when you get there and take them along to the parks to store an extra shirt, sweater, etc. The upside of the cooler Jan weather is that touring the parks is so much easier than in the heat and I think you will find Jan is not that crowded. We didn't make any PS for dinners until we got there and we were fine.
As for OKW, you should have everything you need in your room. If you don't, you can buy almost everything you need or want. I always bring along sugar packets, tea bags and individual packs of coffee so I don't have to go looking for them. Also a kitchen wipe, fabric softener sheets (packed in the luggage so clothes smell fresh) and some extra powdered laundry detergent (double ziplocs should be fine)- again, just to make my life easier. Depending on how long you're staying, you may want to do a grocery store run (there are plenty of threads on this).
Other than that, don't worry - you'll be having tons of fun in 8 days!!!!!