Hi sevenofnine,
I can't share from personal experience with an addict partner, he was a sh*t nonetheless, but I can share my thoughts.
I think that by being honest with them, as you are, and as long as they are safe, they will each develop their own opinion of and relationship with their father.
Perhaps speak to them about a "plan b" should they ever feel uncomfortable, with their dad - such as calling you, to come get them.
When I split from my sons father, many many years ago now, his dad chose to see him once a fortnight, on a Sunday. There were times he wouldn't turn up, or be really late, and it would enrage me no end. My son was only 4 at the time, and it used to upset me, when I saw him upset. As he got a bit older, he had questions that I couldn't answer - and I used to say to him, " that's something you'll need to ask your dad". I also bought him a mobile phone, so that he could contact me if he wanted to come home, as his dad wouldn't let him call me on his mobile.
My son eventually got to the stage where he didn't want to go with his dad - I respected that choice. There was a period of about 2 years, where my son didn't even speak to him.
Like you, I was always honest with him, but explained things in a way that he would understand, listened to him. Over the years, he made up his own mind about his father, and the relationship is poor, to say the least.
Just do what you're doing - discussion, honesty, safety and a get out plan!
Best of Luck
Much Love
Bute x