I asked a similar question on this board and learned that most of the taxis, etc. on GC and Coz. don't even have seat belts, let alone car seats. Therefore, bringing car seats may not help if you are doing excursions on your own. You will probably have more success with Disney's excursions, which I assume use buses with seat belts though it may be difficult to secure a car seat in one of those bus seats. I'd suggest contacting Disney directly. Good luck and have fun!