if the bar or casino is licensed to serve hard liqour for consumption on premises and is off limits to ages 18-20 you cannot carry there period unless you're a police officer.
it really is that simple. if he's letting minors enter his bar he's breaking the law unless they're employees. I don't know what casino you're talking about, what the layout is, what kind of liqours they serve, are they on indian land and choose to recognize WA law or make their own rules? which brings up....
also you mentioned casinos, many casinos are on Indian land and Indian tribes are allowed to restrict firearms as they want on their reservations and set their own rules, so really state law means little at that point unless the tribe has chosen to follow state law.
long story short, without more info there can't be a concrete answer.