Nope! If you are using all Disney transportation you do not need or have any use for a car seat. They will just take up valuable room in your room.
Are your kiddos going to have their own seats? If so I would say bring it for the 18 month old for the plane just to make the flight easier but the other kiddos, no leave them home or use a CARES harness.
you never need them, until one of the buses gets in an accident and flips tossing everyone around...
then you will have wanted one, or someone sues due to no seatbelt as was mentioned above.
We have always bought DD a seat of her own. It was so much easier for all of us to have her in her own seat for the flight. We have flown from New England to Florida and to Southern CA. While it can be a bit tricky to manage in the airport, DD was safe and happy in her seat on the plane.I can't imagine it ever being worth it to buy an extra plane ticket for a <2 year old to have their own seat on the airplane, and then having to lug that car seat all through the airport, and to the hotel, etc. We flew a few times when my son was <2 and we just put him on one of our laps. Yeah, he moved around a little bit and it was annoying at some times, but much less so that having to pay for another ticket and carry the car seat everywhere.
There are no seatbelts on ME or any Disney bus.you never need them, until one of the buses gets in an accident and flips tossing everyone around...
then you will have wanted one, or someone sues due to no seatbelt as was mentioned above.
I can't imagine it ever being worth it to buy an extra plane ticket for a <2 year old to have their own seat on the airplane, and then having to lug that car seat all through the airport, and to the hotel, etc. We flew a few times when my son was <2 and we just put him on one of our laps. Yeah, he moved around a little bit and it was annoying at some times, but much less so that having to pay for another ticket and carry the car seat everywhere.

For our trip in May, our youngest will be 23 months old and she'll be riding rear facing in her car seat on the airplane. The rear facing on the plane isn't as big of a deal as just having the seat in general, but I'll have her ride on the plane in the same manner as she rides in our cars, and that's rear facing. It will also keep me from worrying about her kicking the person's seat infront of us with her long legs. She'll be kicking her own seat instead!
You can bring a car seat on the plane as long as it's approved for use in an aircraft, but it will need to be forward facing.
I don't want to type it all out again, but I'll link you to the information you need:
http://www.disboards.com/showpost.php?p=33320290&postcount=67
http://www.disboards.com/showpost.php?p=33331548&postcount=74
http://www.disboards.com/showpost.php?p=33332573&postcount=75
As long as my child is under 35 pounds (the rear facing limit on the car seat we travel with) she can use it rear facing on the plane. If she were over 35 pounds, she'd have to be forward facing since that is the manufacturer's instructions. If the child was less than 20 pounds (regardless of age), the child would have to be rear facing, as the seat can't be used forward facing under 20 pounds per the manufacturer's instructions. If the child was under 1 year of age, regardless of weight, the child would have to be rear facing because the seat can't be used forward facing for children under the age of 1 per the manufacturer's instructions.
I will always fly with the proper information like linked above because soooooo many people, including flight attendants, are misinformed.

http://www.cpsafety.com/articles/airplaneRF.aspx
More beneficial or not, I prefer her to be rear facing because that's how she rides in the car. Also because she won't be kicking the person's seat infront of her, she'd be kicking her own. And I'd be able to interact with her better since we'd be facing each other.
I was posting the links above because they can not require you to forward face your child on the plane if your child is within the rear facing requirements of the car seat. They also can't make you use the car seat outside of the manufacturers requirements (example: forward facing before 1 year & 20 pounds).
