So sorry to hear about your experience. I know how stressful and frustrating that kind of incident can be. I wanted to write to clear a few things up as a veterinary technician and a dog trainer.
Please don't let anyone bully you into putting this dog to sleep unless they have behavior training and have taken a full history. I did not see what happened and have not taken a full history, but from what you have written, I don't think this dog is a good candidate for euthanasia.
First of all, this dog has grown up with your family and not had any incidents until recently, so you know it has a good base relationship with you. Secondly, it gave plenty of warning signs that I read in your post: growling, acting in a fearful and avoidance manner, hiding under things. Finally, when it continued to feel threatened by humans that seemed to ignore its loud cry to be left alone (whether due to pain or fear or both), it escalated into a snap. Contrary to what someone else wrote, that scratch your dog left on the child's cheek showed a GREAT amount of bite inhibition. Instead of whipping out the equivalent of a gun or a knife (a real bite that leaves puncture or tear wounds), your dog chose to deal with a perceived threat by pushing the aggressor away with the equivalent of a hard shove and a yell ( a scratch or air snap). Dogs have a great amount of precision with where they bite. You dog did not want to maul or hurt that child, she only wanted it to move away from her, and since the child did not understand how she was communicating it (and neither did any adult nearby who could direct the child), she felt the only way to move him away was to escalate into aggression to protect herself. The only significance of it being on the child's face is that it was the closest thing to the dog's muzzle. That is why most children get bitten in the face- they are on face level with the dog.
You need to get this dog checked out for pain first of all, especially since you suspect it already. I would suggest a different vet, if at all possible. The fact that they would say aspirin is the only thing available is just ridiculous. There are a TON of meds MUCH better than aspirin (robaxin, metacam, rimadyl to name just a few). Pain meds can do wonderful things to a dog's temperament when they are grouchy because of pain. Also, the fact that they told you to bring a muzzle because a dog scratched someone with it's teeth?? And that they would slap the label of a "biter" on her without ANY previous bite history? It takes 3 seconds to whip up a muzzle from gauze, too, BTW. Something they should know if they can't even afford to keep any real muzzles on hand. Also, don't automatically assume there is a mental "sickness" here or abuse, any more than a shy child is mentally sick or abused. Abuse is possible, but genetic predisposition is very likely too. Consult with a trainer before assuming those things.
Finally, what I would recommend right now:
1. Keep the dog away from any children until you can come up with a reasonable way to deal with her real fears
2. Assume she is in pain, avoid any unnecessary handling that she does not solicit. If she growls about something, please listen to her! She is saying she is uncomfortable. Punishing, assertiveness, reprimands or retribution will only scare her and make her think she needs to defend herself with aggression.
3. Have her checked by a real vet
4. Consult with a trainer
5. Be realistic about your family situation. If you do not have the time or energy to deal with a fearful dog and all the issues that come with it (management, training, etc.) then look for a new home for her.
Don't assume that just because your dog has done this once that it should be put to sleep. If there was more damage or more bites in her history, it would be an entirely different story, but I don't see any reason right now to put her to sleep. It sounds like a very manageable case in the right home that can deal with a fearful dog. The website
www.fearfuldogs.com has a lot of great info too.
Lots of hugs to you as you go through this. Feel free to send me a private email if you need to.
Sarah Walker
Side by Side Dog Training
www.sidebysidetraining.com