I find it hard to believe the older kids didn't know their parents were using drugs, given they were constantly very visibly intoxicated. In all likelihood the drugs would be in the house if the parents were using them. Nick is drunk and high on stream and apparently drunk and high at church. Kids would know something is wrong with their parents.
Don't know about coaching, but I could see the kids being ignorant of the sex stuff, but I can't see them unaware that mommy and daddy are always fucked up and acting weird.