Hi Mark, I was a part-time FRC mentor last year (I could only volunteer once a week). I assume you are volunteering for FRC (FIRST Robotics Challenge) which is the biggest competition under the FIRST umbrella. FIRST provides a software framework, in both C++ and LabVIEW, that provides a ton of abstraction for operating the robot. It is relatively easy to make the robot run and do things. Some of the challenges of being a FIRST mentor have nothing to do with teaching the students how to program or design a robot. There is a lot of fundraising, logistics, buying tools, herding students, setting up the workshop, setting up a new workshop after you get kicked out of the old one, etc. Being successful in the competition has a lot to do with things like scouting, researching the competition on the internet, human driver skill, and knowing the right strategy for the game itself. It is not purely an engineering competition, although the goal is to expose the students to the engineering field in a fun way.
It is a cliche but mentoring is another case of what you get out of it is what you put into it. I wish I had more time to volunteer, because it really is a fun competition and both the mentors and the students get a lot out of it. I would recommend volunteering at least twice a week, and get involved early (like right now) so that you can be part of the training and preparation of the team before the competition season. I have heard there are a wide range of team philosophies, from very professional and disciplined teams to teams that do 90% of the work over the last week of build season and at the competition. I did not go to the actual competition but I heard it was a fun, exhausting, crazy marathon.
Good luck with your FRC mentor experience!