Chow chow agression

Question
I have a 6 month old chow. He was raised with contact with children. Recently he has been agressive towards some, but not all. He attacted a young girl. What do we need to do to stop this behavior?

Answer
If he was raised with kids, were there any circumstances that made him uncomfortable?  Did he get teased, or was he punished for jumping on them or nipping at them?  I'd like to know more so that I can give you a more complete answer.  It's rare for a six month old pup to be aggressive, even in this breed.  At any rate, sudden aggression in ANY dog is worthy of a trip to the vet to evaluate whether there is a physical cause (vision problems, arthritis, thyroid, neurological problems, etc. can all cause aggression issues).  Once the vet clears the dog, then you should consider having a behaviorist evaluate him.  For now, it makes sense to pair the appearance of kids with good things.  So, if this were my dog, I would not let kids approach (to keep them safe), but I would let them toss a treat onto the ground for the dog, as you say "say hello".  Also, if a child walks by as you are out on a walk, YOU feed the dog some treats while the child approaches, but as the child disappears, so do the treats.  The object is to change the dog's emotional response to children.  Dogs don't generalize well, so if he was exposed to middle schoolers, he may be leery of toddlers, or vice versa.  So, it's important to do the feeding game with all ages of kids.  Just be sure the kids are out of reach, and the dog is securely leashed to you, or muzzled.  Without knowing more, that's about as definitive answer as I can give you, but I think that if you have a dog that's been truly aggressive with a child, or broken skin, that you need the help of a behavior pro.  The IAABC website has listings, or you can check the Animal Behavior Society web site.