"Minor Release Waiver forms" which are required for any racers under the age of 18.

This one is for the parent to sign and is called an annual form so it probably only needs to be submitted once by the racer:

http://www.jonesbrown.com/mts/resources/WPEnglishParental.pdf

This one is for the minor to sign and I think a new one needs to be submitted for each event the racer competes at:

http://www.jonesbrown.com/mts/resources/WPEnglishWaiverForMinors.pdf