Look this is the problem with trying to give context without properly thinking through the example:
The infant is crying for something, give it what it wants (normally food).
Muffle its cries with your shirt.
Put your hand over its mouth and nose to stop it breathing until it falls unconsious, but let it live.
Hit it over the head, again knocking it out but letting it live.
You also have to take into account the parents' reaction if you kill their child in front of them, do you think they'll stay quiet? You just took away their entire reason for living; they arn't going to care about their own lives and they'll want to take away yours.
Just use the damn train track dilema:
A train is speeding down a track and about to hit a large group of people. The track is fenced in: there is no way over, around, through or under the fence in the time it takes the train to hit the people. You can divert the train onto a sidetrack and save the large group, however on this sidetrack is a single baby. If you do not divert the train, the group will die. If you do divert it, the baby will die. Do you divert the train?
You then add other parts to that such as "the baby's parents are watching you" or "you are on camera and peopel will see your actions" to make things interesting. You can try switching the consequences too: the train is speeding towards the baby and you can divert it to hit the large group (this way you don't take action to kill the baby; you let events unfold without explicitly stopping them, there's a big difference to some people).
Now, after that, yes I'd kill the baby every time. One life against more than one life, there's no contest for me. Screw what other people think too, they can hate me all they want but I'd know I did the right thing.
