If complaints are settled at mediation stage, any such settlement is confidential. Only the outcome of cases that go to formal adjudication enter the public domain.
In this case, the employer would be very foolish to allow matters go to adjudication.
While you can fire a person easily in the first 6 months, getting them to sign a document saying they were fired for gross misconduct would be looked on very unfavourably by a labour inspector. If they regularly employ 16 year old they should have policies and procedures in place for training, safety etc.
A parent can complain to the WRC on breaches of the young persons act so I would suggest phoning them to complain. It might trigger an inspection. At the very least I would demand the signed document be removed from the file and destroyed.