The only thing would be to reward him for behaving in a positive manner around children. For example, if he is in the company of children and doesn't bark or growl, just praise him and either give him a treat or play a short game with him. Have you had him since he was a puppy? Was he socialised properly with children as a youngster? You would also need to think about what the children are doing when he growls/barks at them? Are they playing? If so, it could be their actions that are freaking him out a bit. Again, when he barks/growls, is he acting in a frightened manner or does he seem confident? If its got so bad that you can't take him out, then you may want to think about contacting a behaviourist. They may be able to get to the bottom of why he behaves like this and give you advice on how to combat it. In the meantime, you will have to look at how you respond when he does this as you may inadvertently be rewarding this behaviour. My oldest cocker really does not like children but he does not behave in the way that your dog does. He may tolerate one stroke from them but any more than that and he will snarl and snap. He doesn't bother about them at all if they don't bother him so in a way we probably have it a bit easier than you. I really think you should get some help so that you can all enjoy your walks again.