It is cheating if you hold your kid back 8th grade.
If you hold your kid out of kindergarten because he / she is not mature enough, I have no issue. If you hold your kid back in first grade because he/she is not ready to move on academically, I have no issue.
If you hold him/her back in 8th grade, and you are doing so to give him/her an edge athletically, you ARE cheating.But good luck to you nonetheless.
