Knuckleballs are actually easiest on your arm because there's no snapping of the elbow, it's all fingertips, and knuckleballs aren't thrown as hard. Tim Wakefield, the most well-known knuckleball pitcher today, hasn't had any arm problems (I think he's had back problems). Wakefield threw 160 pitches in a game a few years ago but throwing knuckleballs, it's like playing an easy game of catch, since you don't throw hard and you're not putting torque on your elbow.
Every kid pitching in the LLWS is throwing curveballs, some throw sliders (which is hell on arms). These kids are 12 years old. Nolan Ryan didn't throw a curveball until he was 19 years old, and pitched in the bigs until he was 46.