su_A_ve is right. Start with your budget. All rooms are at a premium during that time and it will be very rare to get a promo or special offer.
I would look at the room prices for that time of year and figure out which level of hotel you can comfortably afford. Once you get that, pick the one that suits you best and make your reservation as soon as possible. Rooms do book up for the holidays early and you'll have your best chance of getting into the resort you choose if you book soon.
If you decide to do a package you can switch your tickets to parkhoppers later should you decide to do so.
Keep in mind that parkhopping at that time of year can be difficult because of the number of visitors. We've done it many times, but you have to strategize a bit. The nice thing about hopper tickets is that you can visit a park in the morning and leave when it starts to get too busy, and go to a different park for the rest of the day. We've found animal kingdom to be the least crowded on busy days and it closes earlier than other parks. Epcot may have a lot of people in it, but it doesn't feel as crowded as the Magic Kingdom does when the crowds get big.
If your budget places you at a moderate resort, you might want to chose one that has easy access to the parks you care most about, since your girls are young and may wear out going to and from the parks. I wouldn't worry too much about the pool theming because at the end of December it is usually a bit cool for swimming. We've found the "heated" pools to be too cool for our kids at that time of year.
