I believe that the reason that children are falling away is actually explained in 1 Timothy 4:1 & 2, "Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; Speaking lies in hypocrisy; having their conscience seared with a hot iron."
How do we fall from the faith? By giving heed to seducing spirits and the doctrine of devils. If you look at giving heed part there, what does it mean? The dictionary says that to give or to pay particular notice to or to draw your attention to.
So in this verse, it says that fall from the faith, they giving heed or paying attention to seducing spirits. The word seduce means to entice or to draw you away. So spirits, which you must note that it is capitalized, it is not of God. If it was the Holy Spirit, 1st the name would be capitalized and it would not be labeled as leading us astray.
But so we look at this verse, "In the latter times, some will depart from the faith." A lot of opinions are going around that says that "if they were of the faith, they would not depart from it. And if they depart from it, they never had it in the first place." While in that sense, yes, in a lot of cases, that is correct. I have seen a lot of "professing" Christians that has no outward signs of being separated from the World and no fruits of the Spirit that I could see flowing out of their lives.
I know that we only look on the outward while God looks in them. I am not judging them, nor do I know where they stand at before God. They may once had a true conversion and fall away over time, or they may not have a true conversion and only says that they are a Christian.
But the one thing I want to point out in this passage is "Some shall depart from the faith." If you depart from something, you must to have it before you decide to let it go. If I were to travel to New York from my house, I must leave my house to go to New York. The word "from" means that they were there, but now they are not.
But getting to the topic question about why so many people fall away from the faith. I believe the answer is about giving heed to seducing spirits and doctrine of devils. Young people are simple. According to Proverbs, it means "Untaught." Younger people, especially youth, are attracted to more modern things, like cars, sports, TV, Internet, and so forth more than most adults. And with those addictions, these spirits will come. And if they do not keep on the whole armor of God, they might get wounded with one of the devil's darts that can lead you astray.