We can debate if the assisting 2nd cop acted rationally. From my observation the entire incident was provoked by a belligerent mother, and later escalated by the 14 year old kid that assaulted the cop while the cop was trying to restrain and arrest the mother. The 2nd cop to arrive evidently knew he was responding to a call for assistance, and upon arrival observes the mother and kid fighting with the cop. At this point I think its fair to assume the 2nd cop had no idea what was going on, except for what he observed upon arrival, specifically the woman and kid fighting with the 1st cop. There is no reason to believe the 2nd copy even knew there were kids in the van.
The 2nd cop shooting at the van is a subject for discussion and possible action, partially depending on what he observed upon arrival, or may have known as he responded to the call for backup. I did not see anything that the 1st cop did wrong.
The belligerent woman did just about everything possible to provoke and escalate the incident, and junior further excalated the incident when he jumped in the fray. As a minimum the woman should be booked and prosecuted for failure to yield, child endangerment and for resisting arrest, on top of a speeding ticket. The kid should be arrested and prosecuted for assault on a police officer.
There is nothing shocking about the story. The video clearly shows what happens when you fail to yield to an order by a cop. If the cop is wrong, you can deal with the matter later, rather than the heat of the moment alongside a highway.