Callie I know this has to be so difficult for you. I'm so sorry you have to go through this. I sometimes don't post because I'm not sure if it will help or not but I had to give you my two cents worth ; )
When I read your post for some reason I felt pain for you and your children AND your AH. I don't have children but my ex ABF has children and I watched him be a good dad when he was clean and then the total opposite when he is using. The drugs just take over. I have seen him cry many times because he KNEW and felt in his heart what he was he doing to his children but he could not bring himself to stop because of his addiction. It tore him apart inside and he would just use more to escape the pain of what he was doing to other people and his children.
I think if I had children I would just tell them Dad is sick right now and you may be able to talk to him sometimes and sometimes you might not be able to but he still loves you. That is all you can do. Ripping AH for not being there won't solve the problem because he is too far into his addiction for it to have an impact ....just my opinion.
Take care and hang in there.