Well, my understanding (I stress, my understanding) of original sin as put forth in the catechism is that by sinning Adam lost/forfeited his divine sonship which is why this gift/nature could not be passed to his decedents. The CCC seems to speak of justification/sonship interchangeably - at least when discussing initial justification.
So, while I would personally agree with your statement “once a child of God, always a child of God”, the CCC does not seem to reflect that.
CCC 339 Scripture portrays the tragic consequences of this first disobedience. Adam and Eve immediately lose the grace of original holiness.
CCC 404 … It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called “sin” only in an analogical sense: it is a sin “contracted” and not “committed” - a state and not an act.
CCC 416 By his sin Adam, as the first man, lost the original holiness and justice he had received from God, not only for himself but for all human beings.
CCC 417 Adam and Eve transmitted to their descendants human nature wounded by their own first sin and hence deprived of original holiness and justice; this deprivation is called “original sin”.
CCC 1996 Our justification comes from the grace of God. Grace is favor, the free and undeserved help that God gives us to respond to his call to become children of God, adoptive sons, partakers of the divine nature and of eternal life.
So, if one can lose/forfeit justification it seems the same is true for sonship.