Were formal charges ever brought? It sounds like there were never any charges brought because "his father decided not to press charges."
If charges were brought, and the State entered a "nolle prosequi," the charges may be renewed at any time by the State. If charges were brought, and the case was dismissed by a judge after the presentation of the State's case-in-chief, then charges may NOT be renewed-- as "jeopardy" under a double jeopardy analysis attaches at the beginning of trial.
If charges were never brought, charges may be brought at any time before the statute of limitations. In Maryland, misdemeanors that carry the potential for incarceration do not have a limitations period (second degree assault is a misdemeanor punishable by a maximum of 10 years).
You may and should report abuse to the police. Whether charges can or will be brought will depend on the State's Attorney's assessment of the credibility of your statements, and the strength of the evidence. That the victims may be unwilling to testify against their alleged assailant will weigh heavily against the initiation of criminal proceedings.
They can charge based on new incidents of assault/abuse. You can testify to what you actually saw. The old charge was not a conviction so it cannot be used. A charge needs to based on a specific recent event. A protective order must be based on a fear of death or serious bodily harm and usually needs to filed within 30 days of the incident. HOWEVER you should contact an attorney for a consultation. Many of us offer them free. Good luck.
This response is based on information provided. Many variables may exist that can only be addressed in an interview. This answer in no way creates an attorney-client relationship.