For many Christians parents they are dealing with the education and upbringing of their children as does the world, not as God directs. Rather than desiring their children be born again in Christ, learn to follow and obey the Lord, living their lives for His glory and honour, they want the same things the lost world wants for their children, the pursuit of worldly gain.
Listen to most Christian parents and they sound just like unsaved parents. They want their children to be accepted by the worldly, they don't want their children to miss out on all the worldly things going on in the public schools, they are willing to sacrifice almost anything so their children can play sports in school, their goal for their children is to get them into a worldly college where they can learn how to get a worldly job/career which pays lots of money so they can lay up lots of treasures here on earth.
I've know of a couple such cases and have read of many more where a Christian parents child will get saved and then at some point their child hears the Lord's calling for them and when they share it with their parents their parents discourage them. The parents put much pressure on them to ignore God, follow the worldly path of "success" and then sometime later after they get their worldly college degree and have their successful worldly career, maybe then they can talk about doing what God wants.
If it's important for adult Christians to maintain separation from the wicked influences of the world, how much more important to separate children from such? When the military gets new recruits they don't just drop them behind enemy lines and tell them to do their best. Recruits go through basic training first, and then most go through much more training afterwards. Does it really make sense to drop unprepared children into the enemy camp and then allow that enemy to teach, instruct, mold and conform them?
Visiting public schools (in America) is like taking a tour of a sin museum. Evolution is all over the place, ungodly philosophies are taught as truth, God and Christianity are either totally ignored or belittled, false religions are built up, teen sex is spoken of as something that "everyone does" so they hand out condoms, offer "helpful" information on abortions, immodesty abounds throughout the school, even among the teachers and staff, students kissing and sometimes groping, sinful music blaring all about promoting the wonders and joys of drunkenness, drugs, sinful sex, foul language of the worst sort is heard all about, talk of the latest sinful TV shows and movies is all about, with some being shown in classes, Bible reading isn't permitted but studies on Islam and paganism are to be found, students are taught the biblical account of creation is a myth, most or all of the Bible is myth, that death is simply the end of life, there is no absolute truth, God is an invention of man to manipulate people or provide a crutch for the weak, materialism is promoted as a noble goal, and on and on.
Then if one looks at the textbooks they are filled with ungodly "political correctness", distortions and outright lies.
So, while Christian parents pursue the "good life" here on earth, unwilling to make sacrifices in order to give themselves fully to God and raise their children in accord with the Word, they sacrifice their children to the world so they can gain the world.
Our own selfishness and failure to raise our children for the Lord is why so many children from Christian families are lost to the world.