German Sheperd Behavioral Problem

Question
We have a 4 1/2 year old German Sheperd we've had since he was 6 weeks old.  He has had some obdience training, just the basics sit, stay, lay down, etc. nothing to intense.  Although we personally do not have any kids we've made a point to keep him well socialized with them, he's always been very good with them, and never been nervous or aggressive around them, until the other night.  We had some friends over, close friends who are here all the time.  They have a 6 year old girl who just loves him to death, we were all sitting around watching a DVD, she was on the floor sitting towards the back of my dog petting him, and all the sudden he turned back towards her and lashed out at her, I've never seen him do this not even if someone was bothering him, normally he'd just walk away.  He isn't fixed and now I'm wondering if this is our next step.  We're just terrified to have anyone around him.  No one saw exactly what happened, but we just don't want to take any chances of it happening again, and I worry if and when we decide to have kids. This sounds so awful, but he is really a good dog!  I can't help myself to believe that maybe she startled him or leaned on him wrong not knowing it for him to do such a thing.  I know she didn't mean to do anything wrong and I'm not blaming her in any way, but it's just so out of character for him.  The parents have been so good they both say it was just an accident and believe that it's just one of those things that happen.  But I feel partially responsible.  What do I do?  

Answer
Hi,  First off, you really need to try and find out what happened as best you can.  Even a neutered dog would react if someone leaned on something sensitive or grabbed something.  Normally in a standard dog that has shown no aggressive issues at all and something like this happens it normally points to something happening to them that was startling.  Did he growl at her, nip or bite her?  What exactly was his reaction?  It is sort of like solving a crime scene.  You have to get all the facts and reactions on everyones (including his) part to figure out what might have happened.  Neutering at this age wouldn't change a thing.  If you can give me some more facts of what exactly happened as far as what you can find out, what you saw in his reaction (hair raised, growling lunging, biting etc) I can get a better idea of what might have happened, but it really does sound like he got startled by something she did and he reacted.  It might never happen again, with these situations you never know.  Watching his reactions around her now would be a good test to see what he is thinking.  If he acts suspcious of her, won't let her touch him down his back or anything like that can also give you a clue as to something happening between them.  Especially if he lets others touch him.  If you need more help, let me know.

Dawn