As I said in an earlier post (http://www.momsparentingskill.com/2012/01/best-dog-breeds-for-kids.html), the way a dog is trained and treated determines its personality more than its breed does. Basically, any dog can be nasty if treated badly enough, and any dog can be sweet if trained to be. Simply put, every dog is different. However, there are still some breeds that aren't good for young children due to innate, genetic characteristics. I'm not saying that you should never get any of these breeds as a pet for your children, but you should definitely do your homework before bringing one home.
Pit bulls have been given a bad rap. They are usually very sweet dogs if treated well. They are, however, rather impatient with young kids and tend to go for the throat when biting. That's just not a chance I'd want to take. They can be very territorial, which is a good trait for a guard dog but not for a neighborhood with a lot of other pets and people. They might be fine for teenagers, though.
However cute they are, Chow Chows aren't the best with kids because they are very quick-tempered and have been known to snap and bite without warning. They also have thick, fuzzy hair that kids are likely to pull on, which will NOT be received well! It's not really their fault; their instinct is to be very protective. It's just that Chow Chows aren't easily trained and can be very impatient. It might be fine to have a Chow Chow puppy and get it used to children early, but otherwise they don't do well with young kids.
When it comes to children, Rottweilers are a mixed bag. While they can be remarkably gentle and affectionate toward people they know and trust, they also have a tendency to 'turn' and bite without warning. Like the pit bull, Rottweilers are only aggressive when they have been trained to be that way. Still, their size makes it easy for them to over-power someone.
Since Chihuahuas are small dogs, you wouldn't think they were dangerous. They mostly aren't, but they are very nervous dogs that can be easily spooked by loud noises and a lot of commotion. As a result, they can snap and bite. It's not that they're mean or impatient so much as that children would probably scare them. They're small enough to where kids may try to pick them up, and that doesn't go well on either end.Like I said, any dog can be a good dog if treated well. I'm only mentioning these breeds because they have been seen to be more aggressive than other dogs and implicated in bite incidents. If you get them as a puppies, bring them up around children and also teach your children how to behave, they should be fine. Just do your homework, and make sure to talk to the breeder or shelter you get your dog from about how it has been so far and if there are any particular problems it has.