I spent all weekend playing this game, (a nice length of about ten hours, if you explore in search of items quite a bit like I did) and while I found the crawlers and pack incredibly wrong, well, that's the point. For one thing, as someone mentioned earlier, the necromorphs would not spare children, so the lack of said child necromorphs would be a logical problem, but dead rising, left 4 dead, and other "infection based enemy" games manage to get by without young infected, I'd have let them get away with it too.
The point is though, horror is littered with examples of making a dangerous situation scarier with the use of enemies you wouldn't want to simply kill. Zombies? They aren't truly scary because of getting eaten alive, or their numbers, or even their ability to keep going with gaping holes in them, it was the thought of having to kill your family, and friends in order to survive. What about alma? How better to make an evil force basically unkillable than by making it take form of something so seemingly innocent as a little girl?
By casting children, and even babies, as enemies, dead space 2 taps into that disturbing feeling of having to kill something that typically, a decent person wouldn't even so much as hit. I respect this game for daring to break that boundary in the name of having a more effective and disturbing experience, like films have dared to break in the past.