On the drive to work this morning, I witnessed a dad physically abusing his son in their car as we were in traffic (they were in the car behind me). At first I thought they were playing, like the dad was tickling the son as I do with DD in the car sometimes, but as I kept watching in my rearview mirror, it was obvious that what he was doing was becoming quite violent and sustained with him eventually undoing their seatbelts and seemingly threatening to throw the boy out of the car at the traffic lights (he looked to have only been 7 or 8).
There was also at least one other child in the back of the car in a car seat who honestly looked totally unphased by what was happening 
I made a mental note of his car reg and they turned off towards 1 of 2 schools in this particular part of town.
I rang the school I'm fairly certain the boy attends (uniform colour) and reported what I'd seen. The lady was lovely and noted down everything and said they would look into it, but will the school also report to social services and the police? Or should I make my own report?
I wish I'd jumped out of my car and leathered this dead-beat bastard myself, but we were on a busy road and who knows how he would have reacted. I feel guilty for not doing more though.
*I assume they were father and son, but I suppose he could be anyone to the boy...