One cannot assault another because of words. There are no such things as "fighting words". If one hits another in response to a verbal lashing, then one is committing an assault. If one hits another because one is defending a third party or oneself, then one is acting is self defense.
Because they are children, the police are not likely to arrest or the prosecutor is not likely to file on them for trespassing. In addition, the false allegation of killing the duck is not a legal problem, especially since it involves a child.
I am unclear - is the resident charged with assault? If so, this is wrong because assault requires some kind of touching - whether offensive (Class C) or resulting in pain aka bodily injury (Class A). Now, if someone threatens to assault another, it could be a Class B terroristic threat.
I hope I have answered your question. Post again if you can add facts for me to clear up the confusion.