Valentine, it really sounds to me like you need an aggressive, knowledgeable lawyer ASAP.
I'm not familiar with SRS either, but assuming it must mean state custody. Depending on your situation, this could open some doors for treatment for difficult child. The fact that I had a job with private insurance blocked my difficult child from long-term residential treatment, even though my mental health insurance benefits were exhausted. I did not qualify for any state assistance. WITH state assistance, however, much more is possible, at least in my state.
Are you afraid of difficult child at this point? I think that was asked before, but I never saw the answer. If you are, then your other children most certainly are. It's just possible this could be a step in a better direction for difficult child to be stablized, as long as somebody competent is guiding the process. Certainly loss of your rights to the parent you believe is less willing or capable to handle things responsibly (to put it nicely) doesn't serve anybody's best interests.
Either way, though, I think you and difficult child need a strong advocate and fast.