I think I understand how you feel but I'm not sure what your question really is. So I'm responding as if you are asking what I do about boundaries.
I have a friend that takes advantage of me on occasion, they are a bit oblivious to it, I really don't think they think about it at all, so I do have to say "no" on occasion just to keep the respect in check. I usually just say something like "oh no I can't this time" sometimes I offer a reason, sometimes I don't. just depends on the situation. sometimes my kids don't get it and I have to answer their questions too but most of the time I just say "no"
As far as kids with behavior problems, that's a difficult one, because that is the parents fault not the childs, and sometimes the child just really needs someone to show them love that they can't get anywhere else. So that would have to be something I'd have to really look into, then decide if I can help or not.
one thing I have learned is that I don't have to give other people the REASON for my decision, I just say "I can't" and change the subject or walk away, most people wont ask why, and if they do I still don't have to answer I just say "I can't, I have other plans" and I never answer the question... "what are you up to?" if someone asks I usually just say, "why what's up?" thus never leaving myself open to "can you do this for me"