First off, I am aware that I bear responsibility for this incident and, in hindsight, underestimated how hurt the dog was. I'm posting to try and unravel my thoughts to make a clear, fair and rational decision.
Last night: The dog had been running around in the garden, happily, all day. Lots of children (my own 3 plus 2-3 from a neighbour coming in and out). No issues. He looked exhausted in the evening, but at night time I noticed he was lame on his right front paw. He tends to sleep under the covers and as I straightened my leg he yelped and stood up.
This morning: He was obviously lame on the paw. He was resistant to putting any weight on it at first, but bore weight more as the morning progressed and was able to run when he wanted to, although was lying down for much of the morning (out of character).
1.30 pm - The doorbell rang. Child from next door. The dog jumped at the door (typical) but not his usual jump. I said to the child 'he's hurt his paw today, if he jumps just say no.' We're trying (successfully) to reduce his jumping and he's responded well to training.
As the child walked in (calmly), the dog jumped at her then suddenly grabbed her arm and started biting her. She was trying to push him off while turning away (no hysteria) and started crying. I was simultaneously trying to remove him from her arm.
Once I'd got him away and in a room with the door shut, I saw that her arm had clear teeth marks, already blue and turning purple. He had bitten her through a thick sweatshirt and I'm certain that he would have broken the skin were it not for that. He wasn't 'nipping', he was biting.
I took her straight back to her parents, explained what had happened and my thoughts that he must have put pressure on his lame paw when he jumped on her and somehow thought that she was hurting him. I apologised profusely and said that once she was calmed, if she still wanted to play I would shut him in a room to allow her no risk of contact.
What do I do? One part of me thinks 'dog bit = get rid'. Another part says 'injured dog = stupid owner, not the dog's fault'.
Clearly I underestimated his injury, but what's the right thing to do now?