First of all, welcome to the DIS!
Early to mid December has always been a good time to go. It has gotten busier over the past few years (it is very popular with DVC owners) but I think is still a great time. Probably better than Feb. especially around President's Day. As you get closer to Christmas, it picks up of course.
As far as the tickets, it all depends. We are not hoppers, we find it just sucks too much time out of the day. Many people disagree and enjoy hopping. It can come in handy during very busy periods when the park you are in just becomes unbearably crowded, but I doubt you'll have that problem when you are planning to go. Just remember that each time you switch parks you have to hop a bus or monorail, usually having to unpack and fold your stroller, and then go through security all over again. I would rather not have to unpack the diaper bag and stroller more than once a day! But again, that is our personal preference. It has been suggested before on the boards to just get a non-hopper ticket and if you decide once you get there that you really would like to hop you can pay to upgrade at Guest Services.
I do not think seven days is too long, especially with littles. Knowing you have so many days in the parks will allow you to slow down some while you are there instead of trying to pack it all into four or five days. The price difference is so slight, I'd go for the max days for sure. We used all seven days last January and still didn't see everything. But we were able to slow down a bit and let the kids play on the playgrounds, enjoy the shows etc.
No matter what, I'm sure you will have a great time! Happy planning!
