It is. And children are taken away. The problem is, the state alternative to parenting is not working problem free either. My mother worked in that area(in germany), so I have some insight and can say, in many instances, the children had better circumstances with their junkie parents, than being dragged through foster homes. Siblings torn apart without warning etc.
What most junkie parents need is practical help, so they can have a rehab, not prosecution.
It is definitely a bad influence. Crystal meth is the favourite around here and especially in combination with alcohol it is pure poison.
But it is hard to tell in hindsight, how a children would have grown up different. I know some, who turned out quite well, despite all this. You couldn't tell by the looks and behavior that their mother was drugged at birth (and before and after).
Much more important are stable social conditions. A junkie mother who loves her children and is there for them at least some of the time is way better than no mother and just distant social case workers to whom the children are just a job. (Some are doing a great job with love, but they ain't the majority around here)
But there definitely also are cases of disabilites, that are very likely drug induced.
What most junkie parents need is practical help, so they can have a rehab, not prosecution.