Congress Park, for the short walk over to Downtown Disney (there's also a quiet pool there, but it doesn't have a slide, splash area or food/drink stand as the pools at Carriage House, Paddock and Grandstand (w/out the slide) do); or Grandstand, for its short walk to West Side, first stop on the bus and feature pool...
But all the SSR "neighborhoods" are good and kids that age should have no problem with the length of the walk from any part of SSR to either Carriage House (where Artist's Palette, the main pool, the arcade et al are) or either end of Downtown Disney...