By far the type of creature I struggle the most with in Dead Space 2 is the Pack, the gray children who attack you in swarms. Whenever I use Stasis I only seem to hit one or two and the rest end up swarming me, and then I have problems killing them effectively. What's a good strategy for dealing with these little guys?
Positioning: The Pack are very weak individually and are primarily effective if they can get to you from behind. You want to engage them in a funnel, or with your back against the wall. You are faster than the Pack but if you get trapped, they can very quickly surround you. Know where they spawn and use this to your advantage, for instance stay under balconies if they will drop in front of you. You should not fight other types of enemies in such closed spaces, but the Pack are so weak that you can be approached head-on with little danger unless you need to reload.
Weapons: The Pack will die to one shot of virtually every gun, so firepower isn't an issue. Instead, focus on ammo conservation and reload times, as that is when you are the most vulnerable. The pulse rifle is the best choice, as it works well against individual enemies and groups, but the plasma cutter is also effective. The sawblade is also great, just watch out as it requires being close to the Pack and you can get surrounded. Rather than reload, switch to another weapon, unless it is a particularly long fight or there are only a few enemies remaining (you don't want to waste expensive ammo).
Statis: I rarely used statis for my playthrough, but the idea is to aim at the ground under the Pack. In addition, you may want to circle around as the pack chases you (dangerous), but it will cause them to group up and be susceptible to a group statis or line gun shot.
The final boss fight involves a ton of the Pack. Ammo is the biggest concern, and you should stick to the outside boundaries and never venture to the middle, keeping on the move.
I used only two weapons to deal with the pack the entire time I played. The pulse rifle is very good at quickly eliminating many of them, especially if they are near each other. Even if the pulse rifle is not an option, I actually tend to prefer the other option - use the javelin gun's alt-fire.
Typically whenever they attack, they tend to come from multiple directions. What I would do when swarmed is to use the pulse rifle to eliminate a few in a single direction. I would then run that direction, and switch to the javelin gun. Turn around and fire a javelin into the ground in front of you (but far enough away not to be hit by the alt-fire). What until the first one runs right by it and trigger the alt fire. The electric shock will trigger for a few seconds. During this time, any pack members that run by it will die instantly. You end up with a pile of bodies and loot. Any survivors can be mopped up with the weapon of your choice, which can include physical attacks (they die from a hit).
Using this method, I typically wipe out the entire pack with around 10 pulse rifle rounds and 2 javelins (1 to fire it, 1 to trigger alt-fire).