14 For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband; otherwise your children would be unclean, but now they are holy.

Is sanctified probably does not mean that the unbelieving husband would become holy, or be a Christian, by the mere fact of a marriage with a Christian,  or the unbelieving husband had been sanctified by the Christian wife or that he would gradually become more favourably inclined to Christianity or  that the unbelieving husband might more easily be sanctified or become a Christian, by being connected with a Christian wife.(Ref.JB)

It also means that they are united by the ordinances of God and they have become one flesh. Since on of them is holy the other and the relationship and the children from that relationship are different from that of an illegitimate union. Here they have a legitimate union and the children are also legitimate. But if they separate the children especially will be regarded as unholy . This again does not mean that the children will be automatically saved. Whether it is the husband/wife or the children out of such a relationship, for them to be saved, they have to follow the repentance – faith salvation principle only. But the believing spouse has a privilege as well as an added responsibility in the sense that they have to win their non-believing spouse and the children to the Lord through their exemplary character and life.