>> If the unbeliever wants to leave the believer then he/she is no longer sanctified by the believer and the child would be what?<<
The question itself indicates a fixation on legalism.
“The question itself indicates a fixation on legalism”
Not really, it is just reading scripture in context and comparing scripture with scripture. 1 Cor. 7 says the unbeliever is “sanctified” and the children are “holy” as long as the unbeliever remains with the believer. It is assumed that the believer will live the gospel to his/her spouse and children and they “seeing his/her good works (living the gospel) will glorify the Father” i.e. respond positively to the gospel.