Someone recently told me about his brother who had fallen away from the Catholic faith. He said that he was sad for his brother, but at least he was "doing well". By "doing well" he meant that his brother had a good job, a nice home, was married, and was not in trouble with the law. I did not want to criticize him at that moment; it was a sensitive subject. I, however, had a different perspective.
If all that matters is this life, then it would be appropriate to say that he was "doing well". If we are to look beyond the immediate and realize that we all will spend eternity somewhere, then it would be incorrect to say that he was "doing well". In fact, while not wanting to be harsh, we would have to say that his brother was "doing horribly". Comfort in this life is not more important than comfort in the next life.
Parents will often slip into this error because they want to have something positive to hang on to when their children struggle with their faith. They want to say that the child is "ok" in some way and they know they cannot say that the child's faith is right, so they gravitate to other "good" things. It is a sentimental decision, but not a wise one.
It is because of this painful reality that some parents even choose to go soft on teaching their children about the faith as they are growing up. They will give them the basics, but they avoid any challenging subjects, and soften their description of those "hard subjects" (like sexual immorality). The rationale for this is, usually, for the purpose of keeping their chidren close and not having them wander away from the family. Letting someone fall into grave sin so that they will keep talking to you is called moral compromise and is not justifiable.
It would be better to tell one's children the whole truth about faith and morality even if it means that they will leave home and never speak to their parents again, rather than to have them hang around continuing to sin and have no knowledge that they need to repent. No, you should not be mean about the truth, but the truth must be told. To leave a child blind to his spiritual state so that one can have him hang around is to push him toward eternal damnation. We must speak the truth, as hard as it is at times, so that children (and others!) at least have the knowledge of what is good and right and have the opportunity to repent and come back to the faith.
As hard as it may be to tell someone truth that he does not want to hear, it is not optional. We are told many times in Scripture that to deny the faith (with anyone) leads to one's own eternal judgment. Let us pray for our loved ones who have fallen away from the faith. Let us pray that they see the light of Christ and come back to Him. Let us make sure we "speak the truth in love" (Ephesians 4:15).