my WHO theory (unrelated to that question)
the child is indeed a Time Lord.
the TARDIS recognizes that the Doctor is the last of his kind, and the TARDIS is fixing that -- not by turning Amy or River into Time Ladies, but by generating an embryonic Time Lord within Amy (and possibly within River and any other woman who has been inside the TARDIS since either the Time War* or the closing of where the Daleks' Creator had been hiding)
* = I know what you're thinking: "but then why didn't Rose or Martha get pregnant?" well, maybe they are. we don't know how long Time Lords gestate.
(heck, kangaroos and tardigrades can put their young on pause, stopping development entirely, waiting for conditions to be right)