I think the earlier answers have overshot the mark. But that is to be expected, as I see it, because the framing here is wrong in a way that automatically rejects answers.
People often argue ethical points from the law, ignoring that potential effects of breaking the law are just part of the equation of life. This question has the same flavor.
All ethics is always about what to do in response to non-written rules. Response to what to do in response to written rules is 'political theory'. We, in the West, have lived so long with a standard of reference in a given book, charter, code or set of mottoes that we easily forget this. Social contract theory is not a replacement for ethics. Our decisions ought not to be made on the basis of a collection of written and unwritten rules -- or we are abandoning ethics itself completely. (A Kantian maxim is not a rule in this sense. It is not a social convention, and it has only its own power, not others')
So your real question here seems to be How does one most ethically communicate about genuine ethics in real life?
I would suggest that, ethics not being political theory, the idea of rules is not the right place to start at all. Taking the framing of 'unwritten rules' is presuming that there is a rule and that you know it better than the other person. If the rule is 'unwritten', this is not true in any objective sense. Rules also involve rewards and punishments. So the very idea of a rule implicitly makes a bribe and a threat to this person: you may punish them for their ignorance and reward them for improvement. Besides often being pointless, threats and bribes are always ethically questionable. The idea that you can genuinely make statements about your future behavior is always in some sense a lie.
I would suggest that the most Kantian way of looking at an ethical problem that is playing out in real life is to do what Kantians do with all moral questions: Try to find a solution you could always follow that does not discount anyone's autonomy. The first move here is into either empathy or respect.
Autonomy is about will, and we have only a very weak view into others' will. If we want to think about their autonomy, ll we can do is to try and create a model of their will, which we generally call empathy, or create an abstract model of human will that gives them as much freedom as possible, which we generally call respect (perhaps more properly 'deference').
If you actually find a suggestion that is fair and either empathic or respectful, it seems wasteful not to share it. So the second move is to communicate the thought. Is the right person to communicate with about this necessarily the person with whom you disagree? Often, but sometimes not. Ideas are shared very broadly in many different ways. (The more 'mystical' among us feel they are shared simply by existing, you communicate them automatically in ways over which you have no control. I will skip discussing that, but I can't help mentioning it.)
The right person to speak to might indeed be the person you see has having been treated unfairly, it might be others around that person, it might be authorities, it might be everyone. (Talking to the child about the broken promise may help a lot more than talking to the adult who is disappointing them. At some point we all need to learn how to know when people mean what they say, and that even if they mean it, sometimes they fail. And if, say, this is an ongoing family dysfunction, it might be the siblings or parent of the adult making the promises.)
For Kant, this is a contingent duty. You are really only responsible for your own behavior, and improving your overall situation is something that you do when you can. For Kant, most contingent duties should be submitted to local custom. So this is an intricate set of social skills determined by your social contract, but it has ethical contents as well. No useful social contract about communication is going to permit lack of integrity. Manipulating others for your own benefit is not the same as trying to help. There may actually be a place for both of these, but you have to know what you are about.