The last 3 or 4 years we have gone in October, but before that, we went the week after Thanksgiving for several years. The best thing to do is to be prepared for anything-- weather can change extremely fast that time of year at WDW and in our experience the forecasters have not been that good at predicting it. That week after Thanksgiving, we have been in the park when it was record highs and getting close to 90 degrees, and we have been in the park at
MVMCP when it was 33 degrees. We have had at least one trip where we started out wearing shorts day and night with highs in the 80s, and ended the week wearing sweaters, coats and gloves day and night with highs in the low 50s. You just never know what you are going to get that time of year. I recall one year where we arrived and the people at the end day or two of their trip as we were arriving were talking about how it had been freezing all week and they had been in coats and gloves and then our trip we wore shorts day and night most of the week. The weather bounces around. Our first December trip, the day before our trip, I checked every local weather station and the national weather and they all said that the highs were going to be in the low 70s all week. We packed jeans and thin long sleeved shirts. The forecast was right the first day of our trip and then there was an unexpected heat wave and it was in the mid-80s the rest of the week and setting records-- we were sweltering to the point we were buying cooler clothes. I learned my lesson that year. That year every day was swimming weather-- we have had trips with zero days of what we would call swimming weather. Basically, my tip is do not trust the weather forecasters and pack to be prepared for highs in the 80s or highs in the 40s.
That said-- I have always thought the variations in temperature made for really fun trips (just a real pain for packing). We have never let a cold day bother us at WDW. We just put on sweaters and coats and keep going. It can be pretty nice to be a bit chilly instead of sweltering hot. Plus, since it is the Christmas season, it gives it that holiday feel.
As far as Splash- if it is cold wear ponchos. One of those December trips, we wanted to ride Splash one evening, but it was in the 50s. We threw on our ponchos and went. The line was pretty much non-existent (might have even been during MVMCP-I'd have to look back at pictures to remember) There was another family and I heard their son who was probably around 11, laughing about us wearing ponchos. When we got off the ride, he was soaked, and I heard him telling his Mom that he was freezing and that we were the smart ones and why didn't they wear ponchos. The only time we have let the cool weather in December get us out of a park was when it was basically cool weather with a monsoon. Even then, we threw ponchos on and stayed at MK until we couldn't take it anymore because of wet feet and jeans that were soaked up to the knees. If we had a way to keep our feet dry, we would have kept going. The park was empty, everything was walk on, and DD who was probably 4 or 5 at the time declared it the "best day ever" because I never let her play outside in the rain at home. Other than that one trip, we haven't had much rain on our December trips, so hopefully you will not have to deal with that! WDW is great during the holiday season! Have a great trip!