My opinion on this matter probably doesn't matter as much because I am not an adult mentor, but here's what I've noticed now more than ever now that I'm no longer a student member of the team.
Mentors are there to hammer in a good work habit. They're there to support the students no matter what path they take. They're there to have the students show themselves the way, instead of just telling the students what to do.
Take for example this year's game. Let's say the students want to use direct drive cim motors and a winch to climb, and one very long pneumatic piston to launch the ball, and you as a mentor already know this will not work. Instead of saying "no that won't work try something else" or "that idea sucks please let's stick to feasible options", you would teach them the resources needed to show themselves why that it is not going to work. Essentially teach them what they need to know so they can show themselves that it won't work.
You aren't explicitly saying that it will not work, you are giving them the resources needed to have them prove to themselves that it won't work. You are also providing education so they can use it in the future. This is basically what FIRST is all about, IMHO.
And maybe that's not your problem. But my point is that mentors are there to keep students on task and to teach them. Like it has been said before, each team has a different culture, but fundamentally, to have a good student/mentor dynamic (imo, again everyone's team is different), the mentor needs to practice what is called "tough love"
When I was a student on my team I honestly got quite annoyed sometimes at all the mentors. But now viewing it from an outside perspective I respect the mentors even more for being so hard on us and I truly believe if they weren't that hard we wouldn't have finished our robot on time.
I'm not saying beat them down with a bench grinder, but realize the difference between goofing off/being lazy and legitimately stressed out from working. Chances are if you practice "tough love" you'll see that the goofing off/laziness will soon turn into tiredness from hard work and I think that's a beautiful thing.
And I'm not saying it should be an everyday thing either. You're a mentor after all, not a dictator, but when the students need guidance give them guidance.
Just my $0.05 (to compensate for inflation).