My vote is the Poly as well. CR used to be a king with kids with the gigantic arcade, but it's changed - and it feels busier than the Poly. The view inside the CR is very cool, but you can always hop on the monorail and look at it...it does lose impact after a while.
With the Poly, you also get a great pool - if your child is swimming already. Plus, there's lots for the kiddies to look at, and it's serene to remove some of the mega-stimulation that WDW is renowned for.
As for the GF, I absolutely loved it when I was 20. But given the room expense, it simply isn't worth it for a 3 1/2 year old...not to mention the stuffiness of some of the patrons. My kids are 2 and 7 and I wouldn't take them there now.
For a monorail resort, gotta go with the Poly. Off monorail? That's a different story...