we have minors at all of our projects. usually there's no special consideration, but for formal programs (earn a bike, after school, etc.) we get them and the 'rents to fill out a medical info/consent/waiver thing. never had to use one, but the lawyer in me and the bod feels better if we cover our butts the way everyone else does, our insurance guy appreciates it and a couple of funders have asked about it. i could post a copy of what we use to the wiki but its pretty standard fair.
as to your second point, there's a list of "10 commandments" at the shop that can be pointed to with an "oy!" when a custy gets out of line (again, standard stuff like "let us know beforehand if you're broke", "respect the staff, other customers and tools", and "if you have a question about how we work, ask us"). at the workshop where i work, we get participants to come up with a list of rules and guidelines at the begining of each earn a bike class that they all agree to abide by, and stick it up on the wall for the duration of the class. they rarely miss anything important, and if someone gets rowdy it seems less authoritative when you can remind them that they agreed to the rule they're breaking.