What kind of training have you done with him so far? How do you react when he starts to bark at the kids?
I would suggest finding someone who is familiar with Behaviour Adjustment Training (http://functionalrewards.com/
). The trick is to treat and praise the dog within a certain distance of the object that is causing the reaction (dogs, cars, kids, etc). You slowly work your way closer until that object no longer causes such a reaction. I'm pretty sure the website will explain it better!
Edited to add: What kind of leash are you walking him on? If he's on a shorter leash, he can't chase after children..?