“and were children only as to nature”
jus refers to law and of course civitatem (noun) is the citizen
so the children of slaves were not according to the law, citizens, they were merely the natural children of slaves that held no political status.
The term "quoad naturam" is used in many theological studies of religion & the bible. Here is an example:
By the law is the knowledge of sin, quoad naturam peccati (transgression, sin). There are many things we should never know but by the law of God, though we have some general notions of good and evil. Rom. vii. 7, saith the apostle, ‘I had not known sin but by the law; for I had not known lust, except the law had said, Thou shalt not covet.’ Those first stirrings and secret lingerings of heart and inclinations to that which is cross to the will of God, that they go before all consent of will, and all delight, these things we could never discern by the light of nature.
quoad in law refers to “as it to this (or) as far as”
should read as:
quoad = “as to this”
jus = “law”
so the phrase “quoad jus” = “as to the law”
The final part of that doc reads in English:
“were the children only as to nature, not as to the law a citizen”
May I ask which book you are reading this out of?