I have had kids and as many as 5 dogs at one time in my home for the last 18 years. My kids and their friends are not allowed to approach the dogs from behind, hug them on their backs, get in their faces, lay on them, or disturb them in any way if they are laying down, and those are only the rules that I can come up with off hand. I can tell you that my kids were never left alone with any of my dogs until the kids were at least 10 years old. I can also tell you that my son, who is now 18 was bitten twice by the same dog, drawing blood, despite these procautions. Both times he did something he should not have done, knew he should not have done, and I was across the room and was too slow to difuse the situation. The dog, a 5 year old 95 LB doberman could have had McKid for lunch. Instead he taught the child a moderate but painful lesson in doggy etiquette.
Dog got in trouble, kid got in trouble, and life went on.
Kids are going to do things that they know they shouldn't, especially young kids. And there is no such thing as a kid proof breed, especially a young dog still learning his place in the house.
Yes, it sounds like you did all the right things, and took all the right steps, but sh!% happens. It happens no matter how many precautions and prayers and barriors that you erect against it. The only way you can possibly prevent any other incidences of this nature is to get goldfish, because all dogs have teeth and in the right circumstances will bite.
I hope this doesn't come off as harsh or unfeeling, but this is reality, at least The version that I live in.
Stella