I first came across this line of reasoning on some reddit thread, and I found it quite damning for Christianity. I’m not sure which way of stating it is better, so I’m going to use a few different ways.
First, I state it in a rather complicated way, citing some evidence and labelling premises and conclusions.
P1. According to most forms of Christianity, all babies who die go to Heaven, at least if properly baptized (depending on the specific denomination).
C1. Therefore, out of any ten (baptized) infant souls, ten of them would go to Heaven, or 100%.
P2. According to the Bible (St. Luke 13:23-24, among others), most adults go to Hell.
C2. Therefore, out of any ten adult souls, it is statistically likely that at least six of them will go to Hell, or 60%.
P3. Babies become adults, unless they die first.
C3. Therefore, if the babies grow up, at least six of them will go to Hell.
P4. Heaven is better than Hell.
C4. Therefore, murdering ten infants, thereby preventing at least six of them from going to Hell, is doing them a favor.
Then, I state it much simpler, assuming we are operating under the framework of Christianity. Again, I have labelled the premises and conclusions.
P1. All babies go to Heaven when they die.
P2. Babies who do not die before or while growing up become adults.
P3. Most adults go to Hell when they die.
P4. Heaven is better than Hell.
C1. Therefore, it is better for a baby to die than to grow up.
C2. Therefore, it is morally good to kill babies, giving them a better life.
Finally, I’ll explain it informally, because I’m not convinced this argument lends itself well to formal logic. At the least, I have thus far been unsuccessful framing it under those rules, although I think I have made it clear.
See, if babies go to Heaven, but adults might not, it is reasonable to say that babies are less likely to go to Heaven as they grow up. The only logical conclusion is that it is right and just to commit infanticide. In this way, the infant is assured of entering the bliss of the best afterlife available. Naturally, it could rightly be considered bad for an adult to engage in this behavior, but isn’t the sacrifice worth it?
I don’t really expect a Christian to accept this argument, although I don’t think the Christian worldview can answer it except by denying the first premise, which makes their god out to be a monster, punishing innocent newborns for crimes they cannot have committed. Indeed, the one time I presented this argument to a Christian, he responded by criticizing my formation of it, saying I was not following the rules of formal logic, instead of responding to any of the actual content of the argument. With such a response, it is hard to conclude anything but that he could not respond to the content.
The Complete History of the Universe: A Personal Journey 
Believe it or not, it is not only the critics of Christianity who have presented an argument similar to this. Christian apologists have argued along the same lines, including such prominent thinkers as William Lane Craig:
Can I use that line of reasoning as proof that WLC worships the devil? I really want to. Or proof that no loving god exists, either way.
On a more serious note, why can’t the same reasoning apply to give Hitler morally sufficient reasons to commit genocide? Maybe God told him to kill all the Jews, we can’t know.
Oh, man. I remember when I first saw WLC’s video where the babies and the children receive an infinite good when they’re slaughtered and we have to sympathize with the bastards that slay them.
Right then–I really believed he was an evil man.