Just to clarify a bit, the lunging, growling, barking, pulling behaviors by themselves aren't necessarily indicative of aggression. They could just be an frustrated, exhuberant boxer trying to get to see the kids and being obnoxious and vocal about it. However, the pilo-erection (raised hackles) can be a "distancing increasing signal" the same as a raised lip can, and as such is suggestive of a level of fear or discomfort with the kids. When combined with the other behaviors, the whole package could be a defensive display.
Of course, I'll state again that only someone who has observed the dog's behavior up to, including, and after the incident can accurately say what is occurring and is in a position to offer advice.
It would be interesting to see video or get a more detailed discription of his behaviors. There is often alot of "stuff" that goes on, and the dog is really reacting long before we think they are. For example, when does he really first react (distance of trigger), and in what kind of situations. What is his initial body posture and stance--forward on tip-toes, low and crouched, weight in the rear feet? When does the hair begin to stand on end--at first sight of kids, when they approach? Does it lower if they retreat? Are his pupils dilated? Is his mouth relaxed, lips drawn back, or corners of the mouth relaxed or pursed? Are his ears forward and erect, or back and flattened? Does he offer any signs of stress or calming signals such as head turns, lip licking, whale eye, etc? What is his tail carriage--high and still, tucked, high and fast wag, even, slow steady wag? Does he drool or do his paws sweat? Does he offer any of these behaviors at a certain point and then change and offer different behaviors when there is a change in the distance or actions of the kids? No one behavior occurs in a vaccuum and the only way to get an accurate picture is to observe the behaviors in the context of his other behaviors and as the situation is occurring.
