Before I reply to this, I feel like I have to purge the theory which I didn't really like and didn't want to believe but which gripped me at that vulnerable moment while falling asleep a couple of weeks ago and so took a firmer, dream-like hold than it has any right to. It went that River is a fob-watched Time Lady and the little girl is her, before she became fob-watched, and the Doctor ends up in a nasty reversal of his John Smith situation with deciding whether to let her know, or letting her know and letting her make the decision, of whether she stays as the human who loves him and he loves or becomes the Only Other Member Of His Species And A Girl And Everything but someone else. But as a theory that left so many questions unanswered, aspects unresolved, that it was no cop for anything other than giving me an unquiet night, I don't think.

This, now, this theory I like. For all the Moffly logic you point out, but I think most compellingly number 6, although maybe I just have a narrative irony kink. There's something about the dialogue in that scene that makes no sense unless River knows more than she's letting on in it.