Snaps at children

Question
I have a 4 year old border collie girl who growls and snaps at children. I had her at the park today and two little kids walked about 20 feet from us and sh started growling. Her lips weren't raised or anything, she didn't really have an expression but her eyes kinda got big. And then when they came closer her growl got a lot deeper. Which scared them and I felt bad. So I kinda held her skin on her neck which helped for a second. She stopped growling and the girl was able to pet her. But when the little boy tried to pet her she lunged at him an snapped. It scared me because I didnt know what to do at that point. I grabbed her as if I was biting her and pushed her to the ground. She never submit to me, just had a mad look in her eye. I am so lucky the parents didn't press any kinds of charges against me because it scared all of them. In the past, she lunged at another kid just sitting on the ground but I was holding her leash short enough that timethat it pulled her back. Please help! I know I'm probably not doing it right, or not coming about this the right way. None of my other dogs have ever done this. What should I be doing?

Answer
What you should be doing at the moment is keeping her away from children.  This dog was doing everything she could to tell you she was uncomfortable and you forced her to accept the very thing she was telling you she wanted to avoid!  No wonder she lunged.  Dogs need to have much positive experience with children while they are still puppies, and if they don't, it's often scary for them to have to interact with those little beings who run, yell, scream, and do all kinds of weird (to the dog) things.  Pulling her neck and pinning her were exactly the wrong things to do, and by doing them you have just convinced her that you are unreliable and aggressive and will not save her.  Please, please, please find a clicker trainer to help you - this dog will need to be put on a program of classical conditioning, and possibly counter-conditioning and desensitization to children.  If you cannot afford to pay for training, then please stop forcing her to interact with kids.  If her growls and snaps are ignored, she really has no choice but to bite at some point.  If you allow that to happen, you may end up killing the dog you love, because in interactions where dogs bite kids the dogs always lose.  This situation requires professional help from a good trainer who understands the science.  Good places to find trainers: Pet Professional Guild, Karen Pryor Academy, Victoria Stilwell Positive Dog Trainers, Peaceable Paws, Truly Dog Friendly.