Well, there's two reasons for that. One, the show is still mostly of stand-alone episodes. Second, they already did see her grow up. In order to find Melody as a child, they would literally have to alter their own history.
Maybe a throwaway line about whether it's possible to still rescue her and being told no, but that might be about it.
I guess I need to rewatch the season and put the pieces together a little better...
-We see baby Melody stolen.
-We see a young girl, presumably Melody, regenerate in the beginning of the season. We don't see what she regenerates into, but can we assume she turns into Mels?
-Mels grows up with Amy and Rory, the whole time trying to find the Doctor. She eventually does, and then she regenerates into River.
-River gives up her regenerations to save the Doctor in "Let's Kill Hitler."
-River then begins to study archeology in an attempt to locate the Doctor again.
Does that cover the basics? River's existence is confusing!
I guess my point is that there is a span between "A Good Man Goes to War" and "Let's Kill Hitler" when Amy has no idea the whereabouts of her daughter. She knows that Melody will be safe and eventually grow up into River, but there's really nothing that says that Amy would never see her baby again in the meantime. Baby Melody could always be found sometime between being a baby and her first regeneration when she turns into the child Mels.