I would say Poly or SSR because of the zero entry areas at the pool. That's a wonderful feature with really young kids! They love being able to wade in and out, sit down there to play, etc. SSR has the added bonuses of a really cool Donald Duck themed play fountain area and an extra small water slide that is just right for the pre-school set (in addition to the larger waterslide). One more thing, SSR has great playgrounds - some of the nicest at Disney World. I don't think the Poly has a playground at all. And the villas are really nice with little ones - especially if you can swing a one or two bedroom for the extra privacy, kitchen, and the washer/dryer!
Of course the Poly has the big bonus of being on the monorail line, which is certainly convenient! (Although we did find the bus transportation at SSR to be really, really good.)