
Can you take him out on a regular basis where he can see and hear children playing? Like walking past playgrounds, parks and generally places where there are likely to be children. To start with, he just needs to see and hear them, not come into contact with them.
If he starts to become stressed, ignore his behaviour and just keep doing whatever it is you are doing, ie, sitting on a bench or walking.
I'm not saying this will work but whenever any of mine have started to display symptoms of fear or stress in any given situation, I have exposed them to it as much as possible for as long as it takes to get over the "fear factor".
HTH :)