Why do some children go astray despite having conscious Muslim parents?

Actually, before children even reach school, you have to remember the first school is home school. You have from one year to six years, when they are learning at home, before they even get to school. But lots of  times damage is done in this period, because child psychologists point out that between the years of 1 to 5, a child’s personality, character traits etc. are formed and those ages are difficult to change later on. You are creating in those early years the human being to come and if you are not careful in those early years, make sure child grows up consciousness of Allah, then by the time they hit 7 to 8 years they are into schools and everything else and you make them to do the same Islamic thing, take here and there, to so and so, but the other influences are becoming greater, that you are fighting an uphill battle, you are not going win the battle, it is rare, few manage to get through, few succeed but mostly fail. That is why Prophet (ﷺ) said, “teach your children salah when they at 7 years”.[1] We have to ensure the environment to raise our child is Islamic and he or she has Islamic friends.

Reference:

[1] [Narrated by Abu Dawood, 495; classed as saheeh (authentic) by Shaykh al-Albaani in Saheeh al-Jaami’, 5868]