Do you need the car seats for your trip? If not, you should consider using the airplane seat belts only or rent Cares harnesses. I cannot imagine how you will handle two car seats and two 2 year olds by yourself.
BTW, EBCI is not early boarding.
We are renting a car and my kids are too small for the CARES harness. I am not worried about installing the car seats since I am a CPST, I am mostly worried about getting seats for the 3 of us together.