Some of us are very, very good at hiding our drinking. I used to carry small bottles when I traveled. And in terms of "looking drunk," as the alcoholism progresses it may appear someone is sober when they are actually drinking to keep withdrawal at bay. Which might make them FEEL better, but still unsafe to drive and leaving them with impaired judgment.
I'd keep a VERY close eye on this situation, and if there's even a SUGGESTION that she's drinking while caring for the kids, take action immediately. Remember, she probably feels she's totally OK--it doesn't mean that she IS.