Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear,
Our family has recently received the news that we will be moving to [location]. This is an exciting opportunity for us to [reason for move]. Unfortunately, it means that we must say goodbye to people who have meant so much to us. I am writing to tell you that [Child's Name]'s last day in your class will be [date].
[Child's Name] has thoroughly enjoyed [his/her] time in your class. You have been such a positive influence on [him/her]. [He/She] looks forward to your class and talks about you everyday. I know that [he/she] will miss you, and [his/her] classmates, very much.
If there you have any questions, please don't hesitate to contact me at [contact information].
Thank you,