Right, they may not know the parent is an alcoholic per se, but I would imagine they know something in their home isn't right. My son realized early that Dad was a little crazy, but didn't really know why, but by the time he was seven he found vodka in the garage with his 9 year old friend who informed him what it was. I think the two of them put it all together from there and I could hear them whispering about Dad sleeping on the couch in the afternoon, etc.
One big reason I got out of there is I didn't want my son to think it was normal for grown men to lie on the couch all day, sleeping (if they do not work nights)! It was a terrible example.