Our team is thinking about mentoring other teams. (Junior FLL, and FLL)
We were wondering what kind of work we will be doing to help the little kids and what we would need to know so we can be helpful for them.
Students in FLL love to work with high school students. Junior FLL students may be intimidated but that can be overcome. You need to remember that younger students do learn at a different paces and act differently than you. Just be calm and understanding and use your FRC skills to help guide their work. Don’t be surprised if one day they walk in and have forgotten what they learned. It happens. Never be surprised if one day you walk in and they want to tear the robot apart and start over. It is part of the way they attack the problem and it may be what you were hoping for the last week or so. Have fun and good luck.
My biggest suggestion is to try and remember that you are there to help them. No matter how cool the lego creations/challenges are, you may want to experiment (play) on your own time after you are done with the kids. 
I remember watching the FRC members get more and more drawn into the design, etc. lol
For JrFLL my suggestion is that having your team sponsor/hold/help with a JrFLL Event would be a far greater method of helping at this level of FIRST… the 2 years that I coached JrFLL we had to travel 2hrs to get to the closest events, and we live north of Hartford CT where there are many FRC teams.
At the FLL level, as a coach I would enjoy having one or two helpers per meeting to help keep the students on task. I now have several helpful parents that fill this role, but in earlier years struggled being able to attend to all of the kids issues. One difficulty I’ve experienced in FLL, having first mentored at FRC level, is that coaches and helpers can not provide any of the ideas for the robot or research project. You will need to learn to bite your tongue when you want to say “no, you should do it this way.”
Skills in NXT-G or RoboLab programming would be very helpful. Also, a skill I see lacking mainly because the NXT servos and KISS seem to have elimated the need is the use of gears. Teach the students about gears and how they can provide speed or torque. Also, if you have the budget, buy the LEGO pneumatics kits for the team(available at LEGOeducation.com)-our team had a blast with pneumatics last year. Lastly, teach the use of the sensors in the NXT kit through programming and construction skills . I’ve been coaching for 4 years with some of the same students, with this year’s team finally having heavy use of sensors (yet they still don’t have the skill of higher level programming with variables and wires). This goes along with Al’s comment above- FLL students might not learn at the pace you may expect or hope.
Be prepared to share how your FRC team practices Gracious Professionalism - FLL teams do not interact much with other teams outside of competition day. Think of ways they can practice GP within their team.
Get a copy of the FLL rubrics (see FLL coach’s handbook) and be prepared to act like a competition judge by quizing the students at every meeting about what they have worked on - have them describe problems encountered, how they were overcome, features of their robot design, details of their research project, etc. Getting the students used to answering these type of questions (especially learning to take turns in sharing their answers) will help them to earn awards. Help them learn to be detailed in their answers (ie when asked where they got their information - “the internet” is not a very good answer, rather “the Nation Science Foundation’s website listing information about topic A”. Even better try to find a expert on the topic that the team can speak with - we were able to meet with the pilot and crew of Snoopy One, the MetLife blimp, and climb into the gondola as part of our research this year - it was arranged with a few simple emails that my wife sent out to companies with blimps (thanks again MetLife!).
While these are all things a coach could/would do, they are also things a high school student can contribute to the FLL team (and the coach would be grateful).
Merle Yoder
FLL Coach - Granby Red Blox - 2nd Place CT State Champions Award - Smart Move
FRC Mentor - # 3146 The GRUNTS - Granby Robotics Under No Technical Supervision