The nice thing about SSR is that there are multiple bus stops and multiple quiet pools. Each section has its own stop and quiet pool, except for the Springs which is right next to the main pool so no quiet pool for that section.
I would request either the Springs or the Grandstand. The Springs is the main area, near the main pool, gift shop, Artist's Palette (counter service) and Turf Club (full service). The Grandstand is a short walk from that, has a very nice pool with very cool spray cannon things, its own pool bar and is the first to be picked up and dropped off on the bus route I think. We loved that section.
We have also stayed in the Paddock which is one of the outlying sections and loved it. That trip we had my mom who was pretty ill at the time and my stepdad who was 81. They had no problems walking to the main area, it took about 5-7 minutes to get to the main area at a very leisurely pace.
I would avoid the Carousel section, it is very quiet but it is the furthest out. Not sure how long the walk would be to the main area from that section but again, it does have its own bus stop and quiet pool.
I might suggest that if possible, you rent a car. Given your scenario you might rest easier having that and the way SSR is set up you can park right outside your building. I have never had anything but great bus service at this resort however.
It may just be me, but when I stayed at Boardwalk (right across from BC) I didn't like having to either walk or take one of those slow boats to MGM and Epcot. I prefer having buses. And while BC is close to Epcot and MGM its not that close to anything else. SSR is centrally located so its buses to everything (but DTD) but not a far ride to anything. Give it a try, we love it, hopefully you will too!
You might also post this on the DVC Planning Board, there are lots of SSR experts over there!
Your 4 yo will love that zero entry pool and the splash area of the main pool as well as the Grand Stand pool area. SSR is a wonderful resort!