They[the boys] are smarter than you think and learn at a far faster rate than you or I did. Teach them the basics to an engine[ fire-fuel-compression-air], basic welding/fabrication techniques and hand tool ID and use[6 point-12 point wrenches and sockets], torque values and such, then give each a 3 1/2 HP motor to tear down and put back together , with your assistance and supervision, and watch their faces light up when they get them running again. Most memorable time of your hotrod life or close to it. Add more info as they progress in age such as front end geometry and frame construction. They will let you know when they want to learn more. Besides, you know where they are and what they are doing.