The marital status of the parents does not affect the status of the children. All children are created in God's image and have equal status in the church. Neither civil law nor church law considers the children of an annulled marriage illegitimate. Nor does the annulment imply that the children were not the fruit of a genuine human love. Perhaps many years have passed, children could have been born of the union, but an annulment process is always an examination of the marriage at its beginning.
Subsequent behavior sheds light on the beginnings.
A 20-year history of adultery in a marriage indicates that the person taking the vows had not the intention or had not the capability of fulfilling the marriage vows, and therefore an annulment might be possible many years after the fact. In the first generation of Christians some pagans were baptized and subsequently abandoned by their pagan spouses. St. Paul allowed these marriages to break up, indicating a distinction between a natural marriage bond which existed since creation and a sacramental marriage. A sacramental marriage between two baptized Christians cannot be broken except by death. Although the church tries to prepare people well for marriage, it is possible that some people go through the marriage ceremony and not contract a sacramental marriage.