NHAnn-I flew to Orlando from Logan in Jan with my DD(4), DS(turned 3 on the trip), DD(just shy of 1). We purchased seats for all and DID get 1/2 price fare for DD(1) as I did for our upcoming Oct trip-you have to call the airline-not available on line. We brought 3 car seats with us. I purchased this great carrier for a car seat-
PacBac It stays close to your body unlike the bags. I was able to put the 4 year olds small booster and a cooler in the full size car seat in this back pack. We also used a bag for the 3rd car seat and since we went in Jan we used the bag for our jackets. We used car seats on the plane for DD(1) and DS who was about 34 pounds at the time. It was the first time he flew and he knew that since he was in his car seat he couldn't get up and run around. I am very glad we had the car seat for him. My DD(4) has flown before and she is a very good girl so I knew she'd be fine-as she was.
When we go in Oct we will be using a limo rather than a rental car so we are bringing just a car seat for the now almost 2 year old. We will let the limo provide boosters for the 5 year old and nearly 4 year old who is today 39 pounds-likely over 40 for our trip. Since he has flown once before I think he'll understand that he can't get up and run around and since he's considerably bigger I am confident that he'll be safe without the car seat in the plane.
Just a few more words of advice for a Jan trip. You'll need to be prepared for any type of weather! We had 1 day in the 70's but most in the 50's and some nights in the 30's

BUT that was unusually COLD. Still you'll need both shorts/t's and pants/sweatshirts/jackets. I found it a great help to mail a package to my hotel. I mailed diapers, wipes, a few extra clothes, some toys, and coloring books for plane ride home. I could have stopped at the grocery for diapers/wipes but I knew we'd want to get right to the resort from the airport and found it worth the $10 in postage to not have to worry about getting lost. Feel free to PM me if you have any other questions about traveling with young ones. You'll have a blast seeing Disney through your DGC's eyes!
