About car seats themselves:
"To use it on a flight, the car seat must be approved by either the FAA (the US-based Federal Aviation Administration) or TÜV (a safety certification recognised all over Europe). All car seats which are FAA or TÜV-approved should have a sticker on them saying ‘Certified for use in aircraft’." So, if you do want to take a car seat on an airplane, it does need to be certified for that.
That being said, the answer to 'Does a 2 year old need a car seat to fly?' is no. A two-years-old can sit in a normal seat using the provided seat belt. So you can use your approved car seat OR justsit your two years old without it. Of course, it changes from airline to airline, so if yours requires you to have a car set forthem, then yes, you can and have to bring your own car seats, airline-approved (usually that means 44cm wide or narrower), with you.
The truth is, if the airplane crashes, the car seats won't make a difference, it's more the matter of your preference, for example, if your babies like the car seats, maybe it's the comfier choice and brings a higher possibility for them to nap in there.