Let me just suggest a possible scenario leading to the cause of the headaches.
The child heads the ball. Somewhere in the brain, a minute fault, or even just an unusual contact with its containment structure, causes a defensive reaction: constriction of the blood supply.
This is a life saving mechanism, but I believe it can also be the first phase of migraine.
Whether or not the real suffering is caused by a hormone that releases this constriction, is not relevant at the moment. It is the processed reaction that is the issue.
Another example is the musician that has had to give up the trumpet because of swelling of blood vessels in his or her brain. The horrible thing is, that in most of these cases, the mind will learn to fear the danger, and the very suggestion of the game...or instrument, is enough to cause the clamping.
Then we get to the era of unconnected response.
I believe that the sub-precessing is quite independent of our normal minds and can churn away, fretting about one problem or another, without letting the conscious mind in on the act. This means that the problem starts to occur at seemingly unconnected times.
So, in the case of heading the football, perhaps subsequent knocks were well within limits, but the one rather bad impact has led to a learned reaction.