It's not easy to say difficult things to people. Especially when the people in question have life altering circumstances in play. But I have to do it every day.
My own experience is, be up front with people from the start. Start up like you aim to keep up, in other words. If I know going in to a certain situation what the possible outcomes and/or consequences are for certain behaviors, I have no one to blame except myself if I choose to engage in those behaviors. And way more often than not, my experience tells me that other people are more likely to accept said consequences better if this is done from day one.
Sometimes I have to do things that result in people going to jail, sometimes for many many years. I don't send them to jail; they do that themselves, and the judge (or parole board, depending) gets the final word. No one would relish having to tell another person this, or worse usually, having to tell their mother/wife/kids. It never gets easy to do. But I have already told these people that doing _____________ is going to result in going to jail. Yeah, it's a semi-copout on my part too. I am human. But I try to imagine how I would want to be treated if I were on the other side of my desk, and I would want to know where the lines are. After I explain these lines and their locations, it's up to them where they color.
In this particular instance, potential foster parents should be told what happens if the kids are molested, not fed, not cared for, etc. I would even go so far as to specifically point out that these limits apply regardless of other lifestyle choices such as homosexuality, choice of church if any, all of it. No it wouldn't be a comfortable conversation to have, but it'd be a damn sight more comfortable than dealing with this Mongolian clusterfuck.
That's just me though. Might not be the answer for everyone.