FAA-approved car seats are allowed to be in the cabin and used by your child IF your child has a paid seat. If the plane isn’t full and you have a lap-child, they can still try to accommodate you and allow the car seat on. You can ask at the gate if they can block out an extra seat for your little one. You can always check it at the gate if the plane ends up being full, so I usually bring it along through security, just in case.