I think it is possible
Drawing an analogy, I know of a friend whose FP, EAD, AP were done/recvd and then he got married and filed his wife's 485. So, if you can add one dependent then it should be possible to add another one (the child in this case).
Discuss with a good lawyer, I am almost sure that should be possible (the only thing would be how close is the principal applicant to get approved, if it is pretty close then lawyers might say no). No harm in discussing with a lawyer, right?