I've been restudying the issue of elder's children who fall away. The La Vista website has an article "The Elder and His Children" by Brett Hogland that concludes that an elder whose children leave the Lord would still be qualified to serve, and another article, "Does an elder need to step down if one of his children falls away?" that teaches just the opposite. What is your current understanding?
The Greek word teknon is used much like the English word "children." It can refer to the non-adult stage of an individual or the descendants of a person. It is because of that duel usage that debates occur as to when "having faithful children not accused of dissipation or insubordination" (Titus 1:6) applies.
Brett Hogland addresses the question of older children, those who are now adults and are not living under their parent's authority. His conclusion was not "yes," but maybe. It would need to be a matter investigated. Did the adult child leave the Lord despite his father's efforts? Was it a case of leaving home and leaving the Lord shortly thereafter? Was there long ago a separate household established and the one who was faithful is now unfaithful?
In my answer, I was focused on the question of whether it is proper, just, or fair that a father's qualification for eldership is based on the actions of another person. To which the conclusion is that it is fair and just because of the nature of the position.
I don't think a hard-fast rule can be made because people are involved. When an elder's child becomes unfaithful, the question is if this is a continued pattern that extends back to his time under his father's rule or a later straying independent or contrary to his upbringing. It may not be something that can be easily determined.
I'm of the nature to err on the side of caution. If I'm convinced that an elder's child left after a continued life of faithfulness on his or her own and that leaving was despite the elder's best effort to bring the child back to the truth, then it could be that the man is still qualified to be an elder. It could still be argued that he had brought up faithful children. It would only be after the child later established his own independent home that he became unfaithful. In the few cases that I've seen, the elder usually decides to step down anyway, so it becomes a non-issue. I would prefer that elders step down until they can bring their children back to the Lord, but I accept that there is some debate on the issue and that is why I posted brother Hogland's article.
More often I run into cases where the child never really was faithful to the Lord. The child's behavior was always borderline and after a few years of being on his own leaves the church -- in many cases, it is right after leaving home. In answering that particular question, I was under the impression that one child left the church not long after leaving home. In such cases, I think it is clear that the elder is not qualified and I would have no problems insisting that the elder step down.