Do I tell them about the drinking, and drugs? That their dad is sick, and I asked him to go away until he finds a way to get better? Obviously the 14 year old understands the situation pretty well - and I think she would be happy for him to be gone as he screams at her the worst of any of us - but the younger ones I'm not sure.
I kept it age-appropriate for my son, who is now turning 14. At age 12, he knew my AH was "sick" and needed to go somewhere to get well. He knew just about everything except the nitty gritty details. I will say that when he found out his stepdad who he dearly loved was an alcoholic, this was somehow a game-changer for him and he felt like he'd been betrayed and kept in the dark. That said, I don't know how I could have done it differently based on what I knew and where I was in my understanding of the disease.
I don't think there's a right way to do it, but best practices apply. What Wisconsin said above.