The withholding of information is as much a weapon in many instances as an automatic weapon.Anyone in custody who is not trying to fight back is no longer an aggressor, regardless of what they are being held for. They are a prisoner. Any theoretical terror plans by such a person, when they’re not in a position to carry out a terror attack, do not justify doing violence to that person.
In the example given, the child is not an aggresor (at least by the definition you have given).
However, not yielding the information costs lives and is as much an action of taking lives as if he had the trigger himself.
Given that, I would consider him an aggresor until the information he has is no longer effective in taking lives.