I know DHS case workers are protected by law to a certain extent, but how far does that go?

Most cases I find on the web have to deal with case workers failing to protect a child from abuse, but what if the case worker was the one causing harm to the child? Essentially the caseworker is using threats and intimidation to get the child to testify and say certain things, and this can all be proven with audio and video records.

What would you have to prove or establish to go after the case worker directly?