If using the car seats are very important to you, then you should rent a car.
Even if there was a way to buckle in the carseats on the buses, it would be impractical. There would be extensive delays for the buses at every single stop while everyone "properly installed" their carseats. You also wouldn't have anywhere to store it while at the parks or DTD.
You should also know that even though it's Disney, you will encounter your share of rude people. We always use the buses, but on our last trip we left after the fireworks at MK one night so the buses were a bit crowded. We were near the front of the line for the "next bus" so we were able to get a seat and my husband and I were eaching holding a sleeping child (3 and 6 and very heavy) to leave other seats open for other passengers rather than occupying them with the kids. There was a number of able-bodied adults who were also occupying seats. A family (mom, dad, and 4 small kids) had to stand!!! It is extremely hard to stand on a moving bus when you are juggling the weight of children and barely have even one hand to hold onto anything. NONE of these adults offered up their seats to this family with many kids!!!!

Since we knew they had more on their hands than us, we let them have our seats and we did the moving balancing act with our kids instead.
Before anyone flames me, yes I know that all of these "able bodied adults" got the seats first as they were before this family in line. I just could not believe that there is not a shred of chivalry in a bus full of people. And I also know that this family "could" have waited on the next bus, but it was late and I know those kids needed to get to bed rather than waiting on the next bus.