Just curious what people do in this situation. We will be traveling by plane to Disney, then getting a rental car for our week there. My twins will be almost 6. They are still harnessed but wondering for the trip if we should booster them? We will be traveling with them and a 2 year old so that means 3 carseats. Better to bring them a light weight spare or basic highback booster? What do you do with the booster during the flight since you can't check it and they can't be used on a plane? Does it fit overhead? Does it count as a carry on? TIA