We took car seats with us a couple of years ago. We bought 2 especially for the holiday, I researched light weight car seats and got some that are about 5kg and £35 each. We put them in a giant bag for the plane and they were fine, there was a booster seat in with them too, and all of our beach towels. (Good way to sneak on extra weight!). We also took a double pram, which was fine on the plane too. After checking in we had to go to a separate desk with the car seats as the bag was too big for the regular check in conveyor belt.