Well, technically the Greek Catholic wouldn’t be leaving Roman Catholicism, but Greek Catholicism (of whichever Byzantine-rite Church they happened to be from). How their children would be received back would depend on whether they were entering the Roman Catholic Church, or the original Church of their parents. So which scenario exactly are you looking for?
Remember, Roman Catholic is not the same as Catholic; Roman Catholic is a specific Catholic Church, just like the Russian Orthodox Church is not the same as the Antiochian Orthodox Church, but both are Orthodox Churches with their own canons and boundaries.
Peace and God bless!
For completeness, the relvant roman canons (which do not apply, since converts go to the relevant sui iuris church, and may THEN transfer).
Can. 845 §1. Since the sacraments of baptism, confirmation, and orders imprint a character, they cannot be repeated.
§2. If after completing a diligent inquiry a prudent doubt still exists whether the sacraments mentioned in §1
were actually or validly conferred, they are to be conferred conditionally.
Can. 869 §1. If there is a doubt whether a person has been baptized or whether baptism was conferred validly and the doubt remains after a serious investigation, baptism is to be conferred conditionally.
§2. Those baptized in a non-Catholic ecclesial community must not be baptized conditionally unless, after an examination of the matter and the form of the words used in the conferral of baptism and a consideration of the intention of the baptized adult and the minister of the baptism, a serious reason exists to doubt the validity of the baptism.
§3. If in the cases mentioned in §§1 and 2 the conferral or validity of the baptism remains doubtful, baptism is not to be conferred until after the doctrine of the sacrament of baptism is explained to the person to be baptized, if an adult, and the reasons of the doubtful validity of the baptism are explained to the person or, in the case of an infant, to the parents.
vatican.va/archive/ENG1104/__P2T.HTM
I can’t find the specifics for converts other than in baptism.
Brother David: do not fret it. I should have been more clear as to whcih canons I was citing.