Do you have a good family counselor well versed in addiction assisting in the transition? I ask because the kids already believe something about themselves and the relationships involved that we all know are very complex and dysfunctional.
By establishing a trustworthy counselor that they can voice their fears and what they do actually know already is very wise and healthy. My first traumatic experience with my father acting outrageously and abusively to my sister began at 7 years old. My mother was not very available nor communicative and extremely codependent to our detriment. Back in the seventies families kept the family secrets in a vault ...
What you tell your children should be based on your individual family situation and their maturity and how much they can understand. Its always best if the narrative is the same between the father and mother but when dad is alcoholic this is usually impossible.
I got my kids counseling and they are both healthy and OK in life and relationships as adults. They were 2 and 7 and the counseling came later in their lives when appropriate (Dad was a huge manipulator and always trying to get them to leverage a reconciliation).