Are you looking for legal advice on this or more advice on how to write a friendly but firm letter?
Does your son live or stay with them?
Is she telling other people this?
Does your son know about this?
Have you asked your son about it?
This is a difficult situation to deal with, at his age there could be some coursity.......which is 100% normal and does not make your son anything other then a perfectly normal natural little boy.
Wither he did it or not is a question you are going to have to ask him. Sit him down and calmly ask him if he did this........do so in a friendly open way and explain to him he is not in trouble. Regardless of his answer (Yes or no) let him know that while it may seem like fun the fact is it is not OK to kiss girls at his age or look at them. Keep it simple and to the point to ensure he is not afraid or confused.
As for the aunt, I would suggest informing her that the allegations are just that allegations and your son has a right to live his life without the fear of others slandering his name. Explain to her that you have spoken with your son, and in the future the children will no longer be around each other alone. Let her know that you are concerned about all the children and their mental wellbeing and her telling other people this "story" is detrimental to your child's health and mental well being. Also explain that should the allegations continue you will have no choice but to consider what is in the best interest of your child to stop the slander.
Now keep in mind while a letter may seem easier this may be better dealt with in person. This way you can start off slow and easy and explain your reasons. If she shows reactions that make you think it will stop then you can avoid the mention of taking any kind of legal action.
At his age the courts would not take kindly to her allegations and based on simple looking and a kiss he would have done nothing wrong. Though the easiest route is first to speak with her and try to explain to her why this is important for all the children's best interest. Her own children will carry a stigmata as well so its better to let this go and learn from it.