Sweetheart,
Not sure what makes you believe your children would be better off staying in such a toxic environment even if he is doing better.
Believe it or not, kids really are as resilient as people say. If you are so unhappy your children will feel that energy and absorb it every day. That is not healthy. If your husband is unhappy in his life and addiction, they will absorb that energy too.
And guess what, reality is reality and though I do believe many of our children have lost treasured and cherished innocence way too young, life is life and it deals out all kinds of great things and not so great things. Your children and you can and will recover if you leave and can learn to appreciate a more peaceful and loving environment.
I hope you find the right path for your happiness. If you find that, choose it and take your kids. Your happiness will also be theirs.
Goodluck