The only real dangers of Smartphones are that kids become short-sighted because they constantly focus their eyes on that small screen mere centimeters away from their face, as well as cheap batteries that catch fire/explode in your pocket or while charging.
Would be a different story if they were using tubes inside of them that produce X-Rays, or were operating at power-levels common to military radars. There are quite a few cases of cancer and eye-damage that can be directly linked to working with/operating these radars in Germany and other countries.