So basically you believe the cop (or anyone else who accidentally hits a child) should be open to violence from family members.
And as an aside, I think you are taking the statement "take it easy, I'm a cop" in the worst context possible. You probably think he said it sarcastically when he very well could have been saying it to let the family know he could help the little girl. Or, yes, perhaps he was saying it in response to the violent overatures of the family. Either way, what the cop said did not justify the family's actions.