RE: When do you just say no
I don't think that there are any hard fast rules here.
But if you are given the chance to be a mentor, take it.
A few years back, I noticed a mother and her son sitting on the sideline. I went over and talked to them, and showed the kid around. He was 10, his parents divorced, living with his mom. He wanted to get into RC, and his mother supported that inspite of the costs. I told him what to get at the hobby shop, never expecting to see them again. They picked up the LT-40, engine, radio and then called me. I then spent a year and a half of Sundays with the kid building the kit, covering it, installing equipment, and flight training. Of course, there were also the repair sessions, and the building of a 4-Star 40 as the follow on project. The kid got to be a very good pilot with better skills on the sticks than 90% of club.
For me, I got to see the RC dream again from someone else's prospective, about the same age I was went I got the bug. It was well worth the effort.