I second the BLT vote. You will want to be able to walk back mid-day. Or even twice in a day. Pushing kids past their limit is a sure recipe for success at Disney. haha. Hopefully you will have a good stroller napper and the walk back will be half the nap!
Book your 2 rooms at Poly at the 11 month then modify to BLT at 7 months. I am new to
DVC but I know Sept is a generally low period. Who knows that will be happening the next few years though with all the banked points.
Personally the first time we took our kid to Disney she was 18 months and it was perfect! She was old enough to know this place looks different from "regular life" but didn't know why. I think she sat in her stroller 95% of the time just looking around amazed. We kept joking that maybe we should get her out to walk around but she was so happy just sitting there.