I absolutely love Epcot, but when we went with my 3 year old last year, it was probably the least enjoyed park because there just aren't a lot of rides for them. Really the only way it would be worth it would probably be if you think they would enjoy seeing all the different countries and would be interested in the different types of building, colors etc. Our daughter napped around the time we went around world showcase which was great for us, but with limited time you probably don't want to count on that.
We skipped Hollywood Studios since at that time Toy Story Land wasn't yet open, but are planning on going once it does. We skipped because there are a lot of shows there, but we knew our daughter would only have the patience to sit through 1, maybe 2 at most. On the other hand, my sister recently went with 3.5 year old twins and a 2 year old, and they loved Hollywood Studios. Their kids love watching shows like that, so it was a great fit. By the time you go, they also may open the new Disney Junior Dance Party, which will be a nice break from a regular theater show. It's nice that there will be rides to go on even if the new area will be crowded, but then I'd recommend thinking of how many shows you think your kids can sit through.
For the other full day, I'd recommend Animal Kingdom. When we went we tried to do two half days there that got derailed from the heat, and we ended up missing over half the park! The Africa section has a train that takes you to Rafiki's Planet Watch which is great for this age. There's a nice petting zoo type area, Doc McStuffins was there to meet along with Rafiki, and you can watch the doctors work on some of the animals and there is a slight education component (if you want it). I'm really glad we did it, but with the train ride to get there, it easily takes an hour or more. The Safari also is a longer ride and is absolutely amazing (would recommend doing it early in the day when the animals are active). The Lion King show is also great, but again, a longer attraction. While at Magic Kingdom a lot of the attractions are shorter, we found at Animal Kingdom each attraction took longer but was worth it.
The other thing to consider is whether you can stay at the parks for the full day or need a break in the middle. We were planning on doing full days in the parks, but when we went (last September) it was insanely hot and we had to take a break mid-day. Hopefully you won't run into that problem in January, but planning your days to be a little more relaxed mid-day is probably a good idea none the less. If your youngest is napping (our 3.5 year old is just starting to give it up), try to take advantage of the stroller nap and plan some time around normal nap time where the 3.5 year old won't do rides and can just sleep (even if it's in the park while your oldest goes on some rides).
Sorry if that was information overload, we just started planning our trip back so I have lots of memories coming up!
