I can only give you my view from the "other side". As a child of an alcoholic that drank my entire childhood (and beyond).
Tell them the truth. If they already know he is an alcoholic, that's good.
- It's not their responsibility (I never thought it was mine, but I know some kids must)
- When someone says something to you or promises you something - Actions, not Words. This is a big life lesson and you help them by sharing it.
They need to know about alcoholism, what it is and how it works. They also need to know none of it is about them. They didn't Cause it, can't Control it and can't Cure it.
They basically need to know much of what you know now. Actions not words, how alcoholism affects the mind, that quitting alcohol can be done but it's HARD (and how that works) etc.
Knowledge is power.
Despite them stating they are fine, I would absolutely at least get them some reading material. Al-Anon literature (or Al-ateen if they have different reading).
The only way you can protect them is with knowledge. They are going to be around their Fathers, he is going to say whatever he says you can't stop that.
The only thing you can do is let them know what you know.