You probably wouldn't use the car much, but I'd say get it for a couple of reasons...
1: If you plan on going to any of other resorts as you claim to, that could take significantly longer if you miss a connection, and if you, for instance, wanted to go to a park, then a resort for dinner, and then back to PC, you could have some long wait times. The service is quite good going from place to place; it's when you have to make connections that you deal with the potential misery. Not such a big deal if it was just the two of you, but that brings me to my second point...
2: The kid. At 9 months old, it might behoove you to be able to make a quick getaway for any reason, not the least of which is that the hot Florida sun could really scramble anyone's brains, let alone an infant. Plus, it gives you an option to leave all sorts of necessities in the car, close enough to fetch, without having to lug things around. And if the kid has a tantrum, and you just have to leave right away, I'm sure that even the most understanding parkgoer would appreciate not being on a bus with a crying child. Plus, having the car ont the way to/from the airport, it'll be easier to pull over right away if any sort of accident should happen.
I'm not putting down the bus service at all, I just think that, if your budget allows a car, it's always good for people with babies to get one. It's a prepared-for-anything kinda deal.