Even Amy and Rory - who, let us remember, have just had their newborn daughter stolen - never express any further concern about tracking her down and getting her back from the kidnappers. They just shrug and go "Well, River seems to have turned out okay." and get on with their Tardis adventuring.

Yeah, that really bothered me too. Though it does kind of match Moffat's character arcs in general - see also how Rory and Amy get killed off written out; they're forced to live out their lives in the 1800s and die without ever seeing their families again, whatchagonnado, it's not like that's something you could fix with a time machine. Moffat is mostly interested in characters as representatives of whatever symbolism he's chasing at the moment, not as characters. Which is all the more annoying because he's great at coming up with both characters and monsters, but beyond the purpose he dreamed them up for, we're not expected to think about them.