Have you tried having a meeting with the parents? I know things can get "lost in translation" if you just tell the team (at just about any age!) Tell them the reasons and that it is a requirement to be able to perform. If you are adamant about the rules (and the reasons for the rules) from the beginning it will hopefully just become part of the routine and not be questioned or forgotten. Hair can be a safety issue too - that is another good point to bring up to the team and parents.
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