There may be other procedural steps that can be taken.
How can you sue if you've never been tried?
You are seriously suggesting they want a trial, aren't you?
Upon conclusion of which, they could end up in prison for 30 years. Jury is always a gamble. Many have commented one never knows what a jury is going to do.
By procedural steps I was talking about a petition for an expungement of the arrest in the furtherance of justice because the charges were unfounded due to insufficient corpus and prosecutor misconduct. It would necessitate an opening of the DA's files with respect to his conduct and the substance of the corpus - whether a crime had actually been committed, not the guilt or innocence of the boys.
If your question was in reference to how could they sue Mangum if there had been no trial, there are any number of torts they could sue under.