My kids told me recently that they were confused about why their dad and I divorced because they never saw us fight. And we rarely did. He was sober before we got married, and I simply no longer wanted to be in the marriage. It had nothing to do with his alcoholism--if anything, I resented him because he was too "good" all the time.
I think divorce is always hard on kids, but I would agree that explanations about alcoholism can wait until they are older--particularly if dad doesn't act out when he drinks.
I also think, though, that if kids are well cared for and know they are loved and safe, they are pretty resilient.