Mostly they are operating on priors. The prior probability of a separation being the right thing to do is very high, because they have a _long list of mitigation before they actually can take a kid away. In the case of a doctor-approved immediate physical danger, they are regulated into acting on behalf of the immediate safety of the child while the investigation is ongoing but even that is considered temporary.
The goal of any foster care situation is to get the kids back with the biological parents, so time is on their side, provided they are not living in a circumstance that disallows the kind of attendance and involvement that the state would require to clear a caseworker to re-unite the family. Sadly, many are.
Source: Foster parent.
I was a foster child who was taken from my parents wrongly. A third party (not connected to child services) made a false accusation to the cops, who took me and turned me over to child services without any investigation. Even though I insisted nothing had happened and even though child services failed to produce any evidence (beyond aforesaid hearsay) over the course of their investigation, child services nonetheless fought extremely hard against letting me go home to my family.
In the end, a judge had to order them to return me to my family because they refused to accept that the accusation had been a lie.
In the meantime, I went through three different foster homes. I was a very difficult kid to foster (I cried and screamed a lot, demanding to go home) and so I unfortunately experienced abuse and neglect in two of the three homes. (My first foster home was particularly severe, which was strange because they were otherwise great parents to their biological kids. At least the other abusive home treated their real children equally poorly.)
[Not a lawyer, this maybe a bad idea. But what you’re describing should have consequences for the party that had caused harm.]
But on the same note, I would also consider suing child services for failing to act according to their own principles and mainly for ignoring evidence.
But we do have an example of another righteous institution misbehaving - churches are now paying back for years of children abuse (Catholic priests abusing children). I don’t see why cases like the one above that forcefully separating children can’t go the same way.
It doesn’t really matter that “most reports are good faith”. Most priests are also good faith…
We don't even know if it was an intentional lie or if they genuinely thought I had been hurt. We never found out what they told the cops.