If you are not comfortable covering all the basics of self defense yourself, I recently saw a posting for "Street awareness and self defense" Workshop. It was geared to children who are transitioning to taking Metro or Metrobus on their own to commute to school. This workshop already happened, but maybe you can get into another that they offer.
Key lines from the note I had received about the class:
.... Ideal participants are students who are going to middle school now or next year, and whose parents want them to commute on their own - 10-12 year olds.
... include a brief description of your child‚s commute to school by public transit so that class role-plays are accurate.
The workshop will be taught by Lauren Taylor's organization, Defend Yourself.
She will make the children confident and competent - able to be "aware" and to be effective in preventing unpleasant encounters with adults as well as to obtain the support of adults when they want it. The class will enable them to commute with confidence. It lets students practice effective verbal and awareness techniques, and it does include some physical skills.
The website is
http://defendyourself.org/bringus.shtml