I am not a teacher. Here's my try. But I hope you can try to write it by yourself first next time and post it up for people's comments.
Dear Mr./Ms./Mrs. (Choose an appropriate title with the teacher's surname)
I am the father/mother [choose one] of Peter Simpson's [replace with your son's full name], who studies in Class 1, the second grade (replace the right info.). I am writing for asking a three- day leave from Dec. (Fill in the date) to Dec.(Fill in the date) for my son because we have a special religious celebration -(eaid??) check your spelling.
If you have any question, you can reach me via my phone no. 123456 (replace with your real phone number). Thanks!
(Your Full Name)
PS: Please clearly state which grade and class your son is in. The teacher might not exactly remember whom your son is.