My kids were 3, 8, 10.
I think they had more of an idea of the drinking but rarely was their dad stumbling around or slurring his words. He was crabby sometimes but he was not verbally or physically abusive to any of us. When things came to a head, he went to treatment. At that time I explained about alcholism and the behaviors their dad had (that they could see/relate to) that made him an alcoholic. I should have had that discussion earlier.
When we divorced I told them that adults sometimes have problems that kids do not always understand or see. They are adult issues. Sometimes those problems are so big they can not be fixed and the only solution is to live apart. It has nothing to do with the kids. It is not their fault. I did tell that that mom always tries to make the best decisions for all of us, including them.
Xah spun out of control when I filed for divorce and two things happened. 1) they could see much more erratic bahavior which was confusing for them. The drinking increased. They worried. They felt like I was doing something very wrong. 2) He told them things like I didn't love him, I was wrecking our family, I refused to go to counseling like I was making them do because I didn't care about the family, I was wrong, he yelled at them for being overwhelming and interrupting one night and that is why we were getting divorced, I didn't have to move I was just being mean (we moved 30 miles away to a place I could afford). I suspect he told them I didn't love them but I didn't hear that one with my own ears. He went on long drunken rants towards me that they heard. 3) He went long periods of time not seeing them, not showing up when he said he would, etc. In their little minds this was turned on me at first because I was the reason we were not in the same house. I think they understand this is not my fault now - I'm not really sure. Their dad moved far away in November so perhaps that is the only reason.
I'll be honest. They did not understand. They repeatedly told me it was not the best decision. They fought with one another constantly. They were extremely mad at me. That all did settle down but it took longer then I had hoped. They still think I made the wrong decision but they accept it now. In our house, before the divorce and he went off the deep end, I was the problem in the home. I was angry. I was controlling. I was inflexible. There was no joy. None. This contributed to their sense of confusion with what was going on (and blaming me I suppose).
The entire situation breaks my heart and I regret so much not protecting them from his erratic behavior that ensued after I filed for divorce. I regret very much my own behavior that prolonged their healing and feelings of security. By that I mean I struggled a lot with keeping an even keel and not exploding and slamming things or yelling. I have always struggled with that (still do - my own mother did) but I am focused now, less overwhelmed, and have found security of my own, so I do a much better job of controlling that. I have also brought joy back in the home - which is also not easy for me. Isn't that terrible but I have to think about making it happen or I tend to be constantly serious and miss the small moments to laugh and find joy in the day. I can focus on that now though, and not the crazy, insecure life of alcoholism.
When I decided to file for divorce I didn't see any other choice for me. That isn't to say that I think divorce is the only choice but it was my only doable choice at the time. I don't regret it but I know the kids have paid a price. In our situation I think the price was smaller then that of growing up in the house with a crazy mother and alcoholic father. My focus now is on just being the best parent I can be and pray that one day my kids will understand.
ETA: No sure why I wrote such a rambling novel. It is very late here and I don't have time to edit so I apoligize!