These days you can never be to careful, the first thing you need to do as a coach is protect yourself and your athletes. While cross country is not a contact sport you never know what can happen. By joining either AAU (Amatuer Athletic Union) or USATF (United States Track & Field) you can get insurance should you get injured or one of your athletes get injured and their insurance tries to sue you. These same sanctioning bodies provide insurance to your athletes should they get injured.
I would recommend USATF since they are the official sanctioning body for track & field and cross country in the United States. AAU also has several good packages as well.
Above you can click on the sanctioning body of your choice for a direct link to start your registration process.
NOTE: It is not a requirement to have a USATF or AAU membership to participate in the Florida Youth XC Series or Championships.