x1 is the former child of x2 by law/custom/norm/standard x3.
This word can be used for disowned/ostracized/banished children (as a punishment or negative familial break), but it can also be used in cultures in which a person loses certain familial relationships after undergoing certain rites or fulfilling certain positive/neutral conditions. Additionally, if the child is adopted, or the parent stops fulfilling their parental duties (such as is the case for a deadbeat parent (see: "malternrorci"), or otherwise), or the parent or child dies, then this word could apply.