I'm very sorry for the loss of your daughter; I can only imagine the grief as I also have a daughter.
To directly answer your question, you would not hire an attorney to press criminal charges against those responsible. Prosecuting a criminal charge is the responsibility of the State. I would continue to stay after the local Sheriff's office insisting and potentially even demanding to know what they are going to do about this situation. Also you will want to contact the District Attorney's office and inform them of the situation. There are many times breakdowns in the communication lines between the police and prosecution. It may be something that was overlooked unfortunately. On the other hand, the Sheriff may have determined that there is not enough evidence to make an arrest in this case at which point there may never be criminal charges. At minimum, I would continue to contact the Sheriff's office as well as the District Attorney's office as this is potentially a very serious charge against those responsible.
On the Civil court side of things, you may very well have legal recourse against those responsible. I would recommend contacting a local civil litigation attorney to discuss those options immediately.
I wish you and your family the best in these very trying times.