That's not neglect, if true, that's child abuse & should most definitely be reported... You will be best to offer up the names of the witnesses because police will need to get statements...... If the witnesses are under age, that could be a problem as technically a parent must be present during interviews.... If the abuser/s are also the parents of the witness, then that may change how the police are allowed to proceed regarding interviews.... BUT. Lets not get ahead of ourselves. The first thing to do is report the abuse & take direction from the police
Seconded. What you're now describing is an entirely different thing altogether and yes, you can and should (in my opinion) report it. This is a very serious assault, plus (I assume) threats of assault against the witness(es) - which is witness tampering, also a very serious offence.
Ultimately, the Police will need to speak with any witnesses, so as pointed out by Atticus, if you do report it, you pretty much won't have any choice but to give up their details.
The huge problem that is now very clear, is that reporting this may put the witness(es) in danger. As Atticus touched on, it would help to know if they are in any way related to the alleged offender.