Kids who are in the adoption or foster care system have been severly traumatized for whatever reason they are there. As such they will have serious behavioral and health issues many times that take years to work out. The parents have to be capable of dealing with those long term issues. Many simply aren't. If it happened their own kids will likely tell. Kids have a way of telling you things you need to know but will do so in their own time in their own way when their mind can deal with talking about it.
I have a connection to this family, and I just wanted to say that there are very significant testimony about abuse in this household, coming from the other children in court. There was a hearing that took place a few weeks ago, and everything I've heard (from several unrelated people) that Hana was abused, seriously malnourished and left outside as punishment. That's why she died in the yard, and that's why the other children were taken away. Carri and Larry had seven biological children and adopted two children from Ethiopia a few years back. The children were schooled at home and I was also given the impression that they no longer attended a community church but practiced their religion at home as well. So these kids didn't really get "seen" by society like the rest of normal children do. They existed in a very small and isolated environment, so that may explain why CPS had not previously been involved...
I think CPS might be right this time. As I said, they are not ALWAYS wrong.