I have seen, in higher levels of ball, coaches trying to steal signals by standing close to the foul lines ( I know this is hard to believe, that a coach would try to gain an advantage by bending the rules). In this case, I would definitely move them back into the box. For 'kiddie' level ball, I would probably move the coach back only if he was distracting me or the other participants.

And what rule was broken again?

Peace

Pick one, the coach in this instance was standing next to 1b, it looked like he was more than willing to walk onto the playing field to get a better view of the catcher. I suggested he return to nearer the coaching box.

Under the updates (2008) OBR it's simple: back in the box until the batted ball passes you.