This isn’t accurate. The Church does distinguish between legitimate and illegitimate:
Can. 1137 The children conceived or born of a valid or putative marriage are legitimate.
Can. 1138 §1. The father is he whom a lawful marriage indicates unless clear evidence proves the contrary.
§2. Children born at least 180 days after the day when the marriage was celebrated or within 300 days from the day of the dissolution of conjugal life are presumed to be legitimate.
Can. 1139 Illegitimate children are legitimated by the subsequent valid or putative marriage of their parents or by a rescript of the Holy See.
Can. 1140 As regards canonical effects, legitimated children are equal in all things to legitimate ones unless the law has expressly provided otherwise.
This is not accurate either.
A natural marriage is the term used to describe a valid marriage between two unbaptized persons, or a baptized and unbaptized person.
A sacramental marriage is a valid marriage between two baptized persons.
A declaration of nullity states that the marriage is invalid.
This is accurate. However, whether or not a marriage is sacramental depends upon the baptismal status of the spouses. A natural marriage-- marriage between the unbaptized or one baptized/one unbaptized-- is perfectly valid.