Just came back from WDW in July and we needed 2 car seats when we got there for my DS (1 & 3). We took them on the plane with us. We didn't buy a seat for DS-1 so my DD (5) sat in the booster. DS-3 felt so comfortable in his own seat he fell asleep before take off both flights. We also checked our stroller at the gate - so much easier & no problems at all.
I highly recommend checking at the gate and/or taking your own carseat on the plane.