Thanks gentleman. We are teaming up with the EAA people and the Young Eagles Build and Fly Event!!
If you are going top be flying trainers, and hopefully flying lots of kids, then maybe go with 20 pound, just for the durability. At the KidVenture circles, we actually used either .010" or .012" lines on the TuffBaby trainers and they were powered by the Norvel .061 engines. We could fly them on 42 to 45 foot lines, and that gave good lap times for the instructors to work with. For windy days they had some 40 ft lines. the heavier lines were just more durable, and could take the punishment better. Spectra had not come along yet. I understand that they have developed a new electric airplane, as the collection of Norvels were getting pretty tired after so many years and countless flight, but I don't know what they are using for lines. Judging from what I'm seeing on the stunt models I'm flying, I think 20 pound line would hold up for you.
Have Fun !!
Dan McEntee