Just to make the issue more clear, someone could build a grand casino hotel in Miles City, one with heart shaped beds and disco spinning lights in each room next to the Oasis, and stick a big sign out front that says "NO CHILDREN". Now what are you gonna do about it? That's a very extreme example, but is no different, and I'll give you the answer in advance, it's "nothing". Except hire that attorney guy, who probably doesn't have a license in the state of Montana so you'll have to find another one, to harress whoever to pay a few bucks, and ultimately, that's the cost of doing business.
Then at the other end of the spectrum, you've got some person who only owns one little house, and doesn't want to rent to kids. Just because they don't want crayon marks on their walls. This person puts out a sign that says "FOR RENT, NO CHILDREN". Now what are you gonna do about that? You can throw an egg at the sign, and feel good about it if you get away with it, but the answer in advance is not only nothing, but in this case, less than nothing, because the previous attorney guy is not gonna be interested in your plight since he isn't going to be able to shake out his few bucks. And there is no HUD or FHA police to give a demerit to the person renting that house.
That, unfortunately, is how the world works.