So my Dad drank and drove as mentioned, even had a couple of DUIs. What happened eventually was I had my child and I said to him what if someone who was drunk hit him?
He never drove drunk again after that, that I am aware of. Always took a taxi.
It's worth a shot maybe, although realistically it's probably hopeless. You will have to keep stopping him.
Can you even tell when he's been smoking pot at this point? That might be hard to catch, so a firm no on driving the kids anywhere is probably safer. It's too bad that you have to be the one to police this. But I'm glad you do!