I, too, took my boyfriend back when he achieved sobriety so don't beat yourself up for doing that. That lasted seven months in my case. And like your husband, when he was sober, he was a fabulous friend and a reliable, caring, and good partner. But the minute he began drinking again, I ended the relationship. This was the last straw for me--I never rekindled the relationship--but not for him. He drank himself to death shortly thereafter. Sometimes death is their bottom and no one--even professionals--are able to save them or help them achieve lasting sobriety.
As my boyfriend's drinking escalated and his physical and emotional condition worsened, I separated myself from him completely--at first more to protect my daughter, but then as my head began to clear when the chaos was gone from my life, to protect myself from emotional harm as well.
You are just as worthy of protection as your grandchildren are. You'll find a way to remove him from your home and shield yourself and your grandchildren as much as possible from harm in time. You don't have to act now. The important thing is that you take some kind of action. But you already know that.