There are (at least) two different conditional probabilities we might be interested in here:
1. If a black child is assaulted or killed, who was responsible?
In this case you are partially correct: it is (by about one order of magnitude, not at least two) more likely that it was another non-police black person.
2. If a black child interacts with a particular person (e.g. police, or a non-police black person), how likely are they to be assaulted or killed?
If we are willing to assume (as is certainly the case) that black children have more interactions with non-police black people than they do with police, by more than one order of magnitude, then we can conclude that police are more likely to assault or kill than a non-police black person, conditional on an interaction. It is thus extremely reasonable for black parents to teach their children to avoid police and to try to keep them away from police.