Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [PE Instructor],
Please excuse [Child's Name] from the Physical Education activities today, [date]. [He/She] has been experiencing [reason for excuse]. I feel that [he/she] needs to refrain from physical exercise today in order to fully recuperate. With proper rest, I expect that [he/she] will be able to fully participate in all activities tomorrow. If the problem persists, I will be taking [him/her] to a doctor tomorrow.
Although [he/she] will be sitting out of the physical exercises, [Child's Name] should be able to study or perform any non-physical tasks you require. If you have any questions, please don't hesitate to contact me. You can reach me at [contact information] between the hours of [time available].