I am going to leave Japan soon and I need to say farewell to many people. One of the people I know is a little girl who is about 6 years old. She is very cute and very close to me during the time I stay here in Japan. Now, I want to write a letter to her to say goodbye as well as to tell her to be healthy, that I will remember her for a long time. I am really not sure how to write it properly in this case. Because I think I should use informal Japanese, just like a brother talking to his sister. I have learned Japanese for a while but mostly in formal and respectful forms like masu, desu.
Can you give me some guidelines for writing this letter? How should I express myself in the letter (boku, ore, watashi?), how should I mention her (anata, A-chan)? What useful phrases I can use here?