S.D.
You definitely aren't over reacting. A friend of mine went through something similar recently. They got counseling and he stopped using for a little while. Shen he started back up, she came home one night to fine him drunk/high, passed out on the couch. She called the police and left!
You owe you children a stable, safe, drug-free environment and if he's not willing to give that to them, he's not ready to be a father.
Good Luck!