sevenofnine

Backstory: My xah would be sober for long periods, then fork everything up with a relapse. That would generally take the form of his stealing/selling possessions, disappearing for days while he did coke/meth/crack. Then he would return, sad, remorseful, ready to comply with any program, or take any measure to win me back (what a hero. Barf). He would usually stay sober then for a period of 18 months to maybe even 4 years. And this pattern has continued after we broke up 7 years ago, with the one exception- he didn't disappear or steal money. I guess he is sparing his new girlfriend that, at least.
So here is the issue. He recently relapsed, and my kids (10,15) ended up leaving their dad's house at 1:30 in the morning when their stepmom found him doing lines of something in the basement. She initially kicked him out, but has since let him move back in, 2 weeks later. Overall, I think I do trust stepmother to keep my kids safe (her job is literally in child protection). She is very naive to addiction, so obviously I have worries about her repeating my mistakes. She sounds like me, way back when I was in the throws of it all.
But for right now, at her request to xah, the kids are with me and only seeing their dad for supervised visits. And I am grateful for that support.
My question is how to move back toward 50%50% custody again. I don't think my kids are at risk of finding paraphernalia or drugs, or of being neglected, or any unsavory stuff. The threat is simply the disruption, anxiety and insecurity of their dad possibly(likely!) messing up again. My kids have been really stressed lately with this recent relapse, and I can't tolerate the idea that I just have to send them back there to be disappointed again.
I am not looking for legal advice on how to keep my kids from their dad. Even if he has faults, the relationship is important to my kids, so I cannot completely detach from him.
How do I prepare kids for the possibility of a relapse, without making them get more worried/feel the need to fix etc.?
Xah is able to cover up his use extremely well, and having Iived it myself so many times, I can't expect his girlfriend to be any better at catching him. Nor would I want her all wrapped up in policing him anyway; that ain't healthy. So it is likely the relapse will come seemingly out of the blue, when things feel fine. How do I make peace with that uncertainty, the potential danger of implosion? At some point we will build back up to overnights etc, and back to normal and how do I be ok with the looming threat to my kids?
I can't change who my kids' dad is. So best case scenario here is that he goes back to his 1-3 years sober, then two days trashed, repeating cycle.

Got any advice on raising my kids when we are forced to navigate these traumatizing episodes?
I tell them honestly all of the situation that I know (to the level they are developmentally at, of course, and excluding much of my own anxiety so I don't burden them). But how do I help them safely navigate their own relationship with a addict father?
Or is the relationship doomed by the uncertainties, the 7 days out of a decade when xah royally forks it all up, making the other times when he is a good dad irrelevant because it is a house of cards?