My two pre-schoolers (almost 4, almost 5) sometimes play rough with each other, e.g. when fighting over some toy. When one of them asks us parents for help, it's sometimes part of the game (they want to "win" or gain an advantage over their sibling), and sometimes they really do need help, because they don't want to play that game any more, but their sibling won't leave them alone.
I usually avoid interfering in the first case, but I do want to help in the second case. So I want to make a house rule that when one of them says a certain phrase, the other one has to stop, disengage and leave them alone or face a penalty.
My current favorite is "Dear X, please leave me alone!", because it is polite but still unambiguously states what the child wants the other person to do. Another option would be "I don't like this", which seems more vague. Apart from the immediate effect of helping me decide between playful and earnest pleas for help, I also think it would teach them something useful for dealing with other children (setting boundaries and respecting the boundaries of others).
Is this a good idea? Are there other pros/cons regarding these (or other phrases)?