Spin it any way that you like… if you hold your kid back or reclass them you are gaming the system. You are holding them back because you believe that doing so will help your child. You believe that it will give your child an advantage. You believe that your child can not compete with the children in the grade / class that has been established for children in a particular range of age.
Parents have the right to hold their children back in school but club lacrosse should absolutely use a calendar year age bracket/classification.