The heartfelt sincerity of “The Wizard of Oz’’ - the story, the iconic characters, and Harold Arlen and E.Y. Harburg’s sumptuous music - makes this fable feel fresh no matter how many times we’ve has seen it. The Wheelock Family Theatre’s current production improves on the basics by assembling an impressive cast of both children and professionals (including three Broadway veterans) to populate this magical world of munchkins, Winkies and flying monkeys. Director James P. Byrne (no relation to this writer) and his production team rise to the challenge to be true to the beloved classic film without encouraging too many comparisons.