We see this around here so many times....and then the questions come...
"Well, was the dog excited or 'mad'"
"Was the dog playing?"
"Has the dog ever been like this before?"
And on and on.
A lot of times the poster will downplay the "nip." They'll say how FIdo has never ever acted like this before, how Fido has always been great with the kids and OP THINKS the dog was just playing, but wants to know what to do from here on out.
A lot of posters take this with a sigh of relief, encourage NILIF, upping obedience, and managing there dog, and that they should probably try and "find a 'good' trainer."
It is my humble opinion, that when children are involved, OP's "reading" of the dog should be put on the back burner, and the only advice should be to keep the dog completely away from the child and consult a professional with experience in aggression and successful obedience/behavior modification programs.
I know people hate "running people off," but with this type of situation I think the advice needs to be very blunt, and very serious. They are already diminishing the act to the point that they are consulting strangers on the internet instead of seeking a trainer. I think they should be strongly encouraged to put down the computer and consult someone, "irl." This is just my personal opinion, anyone can agree or disagree.
The reason I HATE these threads is because of the question, "well was the dog just excitedly playing?" How can anyone trust someone who is having their child growled, nipped, or even bitten, to be able to properly read their dog. Sure, it could be just an excited puppy running by and nipping. However, it could also be a dog completely fed up and the next move is a bite.
The following came across my newsfeed and led to this post. Listen to the giggling, and look at the danger this BABY is in. The video taper honestly thinks the dog is playing....every time someone posts about their child and a negative response from a dog, and it's dismissed because OP says, "actually...yeah, the dog was excited and I think they were playing" think of this. These parents not only think the dog is playing, they are so secure in their "reading" of the dog that they posted it for the world to watch and giggle along....