The likelihood of it being "cold" is low.
It CAN get cold, and it's worth having clothes for it just in case, but it's not something you can plan on. Even during The Cold Snap of December 2010 by the 15th it was in the 80s again if memory serves (it was our departure day). And while that cold had happened the year before and again in Jan 2010, it's not absolutely normal.
Prepare for cold just in case but don't expect it.
For our first trip, in 2010 when DS was 6, we stayed 4 days at Universal and wanted to stay longer. And that's with me being a chicken, DH being of a size that doesn't allow him on the big rides he longs too do, and DS being too small or scared if some of the bigger rides.
So I say "go for it". If for some reason you don't love the parks like we did, there's mini golf and other things to do. Stay onsite and you get a lovely resort experience. And so on.