Morality, for me, is based on practicality.
Situation 1: baby cries, guards alerted, baby AND all others die, presumably painful-ish deaths.
Situation 2: baby is smothered, guards not alerted, all others survive.
The morally correct choice is #2. The baby was going to be killed either way (at least according to the OP's scenario), and at least in option #2 it's a "mercy kill," comparatively painless compared to what being shot could feel like (depending on how many bullets and where).
Alternate situation, for thought: I am faced with a decision. Either my entire group including myself is killed, or only I am killed. Obviously, here, the moral choice would be to sacrifice myself for the group. Back on topic, as the parent of this baby, I must do my duty as a parent in showing my child what the moral choice is. By murdering it.