@newleaver1
You might need to re-read spme of what I said.
In any case, to answer your question, this is not about shunning underage children. As you say, parents are responsible to bring their children up. So if a child is disfellowshipped, or the parent of an underage child is disfellowshipped, that does not interfere with normal familial relationship. The parents are still responsible to provide and take care of the child. (The same goes for the husband and wife, if one of them is disfellowshipped they are still husband and wife and act accordingly). But when the child is of legal age, and moves out, then the shunning may begin because the parents are no longer responsible for the young adult.