People have pieced together a fair bit one way or another - most of it has later come out in the media (e.g. her aristocratic background) but her previous children have never been reported on in the press.
Getting drugs requires access to contacts and local networks to put you in touch with a local dealer. They've moved around so frequently that even if they were drug users they couldn't have kept up with their normal dealer, and have been trying to stay away from others as much as possible, so it would be harder to find new networks and dealers.
Of course if they were merely alcoholics then the issues of supply don't apply in the same way.
... And to be honest I think what we can read between the lines about coercive control (e.g. her becoming estranged from all family and friends shortly after meeting him), the chaotic lifestyle (reports of leaving multiple flats in a terrible state and moving on frequently) and CM shouting at the point of arrest that they should let MG go because he has MH issues provide a strong hint of good enough reasons why prior children have been removed, even without substance misuse issues.