To be honest, the waterpark busses from the Disney resorts was convoluted enough before, now even moreso with multiple stops and even transfers if I read correctly...even if I was onsite I would drive to a waterpark for sure.
We honeymooned at the Poly and do dream of staying again some day, but the space thing is just a priority now that we are a family and not just a couple. We've done pretty much everything from values, moderates, deluxes, DVC, our own RV at Fort Wilderness, and most recently WBC. For us, if we can't drive down in our RV with our own kitchen, beds, etc (and our dogs
) then WBC is our next pick. You can't beat the extra space, the convenience of laundry, and the nice resort amenities, and the location is pretty much as on-property as you can get while not technically being on property.
If you're super dreaming of staying at the Poly, how about a couple of nights there followed by WBC? I just think with a family of four, you'll appreciate the space.