FAQ #2:
Does getting an annulment make my children illegitimate?

From Marriage Tribunal, Diocese of Prince Albert:

No.  The children of a couple who obtains a Declaration of Nullity are still legitimate because the the Declaration of Nullity does not destroy the legal validity of the marriage.  It simply declares that in the eyes of the Church, the marriage was not sacramental because the essential elements were not there and no true bond was ever established.