We just flew to CA from Calgary (a 3ish hour flight) with two 4.5 year olds and a 6.5 year old and they were all good in the plane seat belts. We did not need to bring car-seats as my parents were there in their RV and brought seats for all our kids, but we have brought seats on flights before. I think it would be a lot easier to check a seat than to carry it through the airport and security unless you have something compact or foldable like a radian. The stuff we needed to carry on (a bag of toys, snacks, colouring books and pencil crayons, a camera bag, our laptop, and a diaper bag for our 3 month old) made it hard enough without car-seats since it all has to go through the x-ray machine along with you shoes and any jackets or sweaters and any of your lotions, creams or medicines in a baggy. It was tough to manouver, although I know it is necessary. We gate checked seats previously as well as checked them before in a bag and I think they are safer being checked in a good quality car seat bag rather than gate checking, they really do some damage gate checking things in my experience.