I came from a background with some abuse, and hold the protection of children as my responsibility, with certain parameters.
Closing to verbal contact distance would certainly be my first priority, combined with thoughtful intelligence-gathering. Casually finding a way to obstruct the vehicle's escape route in a nonconfrontational way would certainly be another choice I would hope to employ, especially if I were fortunate enough to be with one of my buyu, such as Spartacus. If this is a relative or family friend sent to retrieve the child, I wish no hard feelings generated, and certainly no alarm. I put a lot of stock in gaging people's reactions and demeanor. Placing myself in a position to intervene but not to appear to threaten would be another, important priority. If this is some nutso, I would prefer to be the one threatened. The child should have the rest of his life to get into trouble.