If you had told me a few years ago that 150mm cranks were for children, I would have agreed entirely. Now I would have to disagree. It started with some German recumbent builders putting on short cranks (down to 110mm) as a way of reducing the size of fairings, to improve aerodynamics. But they discovered that they were going, if anything, better with the short cranks, even on their training bikes without fairings.
This has caught on in the recumbent world, and now I run 150mm on all my bikes. It is better for your knees, lighter, and the higher RPM is easier on the transmission, and your heart and lungs. You should also get more power when going flat out. Small really is beautiful!