OP - there are always a lot of difficult questions and situations when you have an alcoholic in the family and children to consider, even years after you split. 10-15 years ago I got great advice and support on the Relationships board here and would recommend you post there for input on the wider issues of alcoholism and family rather than AIBU.
Logistically, when you have a child, it can seem the connection is ever-present. At the same time, you will have to emotionally distance yourself from your ex and his problems if you're going to stay sane and live your own life. Yes, his drinking may kill him and there's absolutely nothing you, or your children, can do about that. It's pitiable but beyond your control.
With DC I've always gone for age appropriate truth and never hidden the basic facts, which are corroborated and understood by wider family on both sides luckily. I've made it very clear that none of what happened was their fault and that they have no responsibility for the way their father behaves.
I tell DC that they probably inherited all the good stuff from his side and none of the bad (but I'll always be on tenterhooks with DC around alcohol tbh).
My DC were so young when we split that there is no memory of him ever being here with us. So for us, it's more explaining why he is the way he is now, why he never visits, why he never remembers birthdays or Christmas, why when we do see him (during visits to wider family) he smells funny, talks funny, hides bottles around the house, and so on.
Occasionally when smaller, we've had tears and frustration that DC's dad isn't like X's or Y's dad who is always reads bedtime stories, takes them to the park, and bakes them birthday cake. After cuddles, we've usually gone through all the other families at school and identified a whole range of people with good dads, bad dads, no dads, and everything in between.
At one school there was a group for children from divorced and blended families. DC was invited to join and the staff noted a mature attitude and sensible advice to another child with an alcoholic parent.
Unlike you, I did always know about my ex's drinking but we were relatively young when we married and I didn't realise it was alcoholism, or what that really meant, until I was expecting DC. It was only when we got to our late 20s and everyone else slowed down with drinking and partying that I realised that exH couldn't or wouldn't stop.
It was then a couple more years before I realised that for the purposes of protecting and raising DC, the distinction didn't really matter. He was unsafe to take care of them (brought home to me starkly when a babysitter declined to leave DC in the house after her shift until I returned late at night, as he was drunk and incapable), made himself unable to work to support them, and was spending money from my account at a rate that would have bankrupted me if I allowed it to continue.
I've been through the vomiting blood, liver damage, seizures, brain scans and the pity of countless medical staff during our marriage. He was diagnosed as an alcoholic in three separate countries so no ambiguity. I've shouted, wept, pleaded, bargained with him. Nothing I said or did ever made the slightest bit of difference to anything. Every single time exH celebrated each small recovery with more drink. I'm guessing he still does but no longer ask.
I'm still sad about such a wasted life and wasted potential but it's largely depersonalised now.
I suspect that one day I will get a call from his family to let me know he is in hospital and probably dying, and I will have to make the hard decision about whether or not to get DC on a plane to see him one more time. If they're old enough, it may be their decision by then.