My dh and I mentored a team when our kids were in elementary school - there are a lot of up sides to FLL, but there were also things that were frustrating for our children and for us as coaches (from our side, the frustration had to do mostly with school policy rather than FLL league). I'll just mention the challenges we ran into so you can know to watch out for them:
1) Our kids were interested in FLL for the robotics, and they loved that part of it. They thought the team challenge part (where you have to complete a non-robotics challenge together as a team) was ok, but didn't see it as exciting enough to be worth being on a team for. They weren't terribly interested in the project presentation and would have been much happier if they'd never had to work on it at all. In other words, my kids wanted to do robotics and only robotics.
2) Our school required us to let any student who was interested to participate, and we weren't allowed to remove anyone from the group for any reason (ANY reason). This meant we ended up with a large group of students, some of whom were very motivated, many of whom were signed up because it was free, it was after school, and therefore put those two together and you have free childcare 1-2 days per week. We also had a few kids who signed up because their friends were there. What that meant as mentors was that our time (mentor time) was spent not just on mentoring but also on a lot of crowd-control and babysitting kids who had no interest in either robotics or the team project.
3) Our school did not share information from children's 504 plans or IEPs with the adult mentors (we did have a sponsor teacher who should have had the information but it was never clear to us that she had any knowledge of it). I personally am torn on whether or not that's info I'd want another parent to have if it was my child, but it did, quite honestly, cause us some challenges which could have had not-so-great outcomes. I don't want to go into the details of other children's business on a public forum, but the issues revolved around children with behavioral challenges which we weren't aware of the root cause of, and hence I, as an adult in charge, addressed the students in a way that would have been appropriate for the behavior coming from a neurotypical child but not for the situation with each specific child with the 504/IEP.
Other than those three things, it was all around a good experience for us, and even though my kids didn't like the project I thought it was an ok thing to include - just a bit of a gotcha for kids who are really going into it mostly for the robotics.
Best wishes,
polarbear
