[watzizname]

Might it be that River is Amy and Rory's Daughter, and the little girl is in fact River and the Doctor's Daughter?

The scene near the end, where River kisses the doctor expecting him to reciprocate, and he is surprised by the sudden development in their relationship, she seems more than a little surprised by his standoffish behavior.

Might this indicate that her and the Doctor did get close (even if we didn't see it happen) at some point in this episode, but events involving the Silence had somehow wiped the memory from his mind?

If they did get close enough, and got it on together, and (If) River is now pregnant, this could go some way to explaining who the little girl is, and why she would be able to regenerate.

The photo of Amy holding the baby, put on display, and surrounded by other photo's of the little girl would also make sense, if Amy is the little girls grandmother.

Sounds a little Peter Pan like, but River could be a future companion of the Doctor's, and someone he falls in love with, sometime after Amy and Rory have left to have their baby?

It's possible that for reasons known only to himself, the Doctor kept an eye on Amy and Rory after they left, so would explain how he knew all about young River, when she first meets him.