The documentation for drupal_render() is quite detailed, but it speaks of elements "having children" without saying what that means. I don't think this is actually made clear anywhere (at least not in the doc element_children() or drupal_render_children()). Even if it is, the documentation for drupal_render() should point there.
(A "child" of a Form or Render array is simply an entry whose key does not start with '#'---and hence whose value is not the value of a Form/Render API property, but rather holds a "sub-element" that (typically?) is a Form/Render array in itself.)
PASSED: [[SimpleTest]]: [MySQL] 33,305 pass(es). View
PASSED: [[SimpleTest]]: [MySQL] 32,875 pass(es). View
PASSED: [[SimpleTest]]: [MySQL] 33,660 pass(es). View