Do you have court orders?
Apart from the smoke thing do you have any other concerns?
Just a thought - go speak to the doctor. Ask for the kids to be drug tested, or at very least find out if the doctor could sort that for you. That way you could write to her and say:
Dear ex,
Please be advised that you're concerned about some of the info the kids have communicated and that if it were to continue, you would consider having the kids drug tested and if it came back positive you would be raising it with the authorities....
One thing to know - if the doctor agreed and a drug screen came back positive, the doctor would have a legal obligation to notify the relevant state authoriites. Hopefully, it won't come to that and she will just have a chat to the boy about smoking drugs near your kids.