Some people have expressed some concern about it, but I don't think it's bad as long as you don't twist the wrist. My biggest question is whether a younger kid's hands are big enough to throw it properly.
The Inverted W is something that is advocated by some people (e.g. Paul Nyman) as something that supposedly improves arm action and thus velocity. I don't think that's the case. I think making the Inverted W -- as Mark Prior, Anthony Reyes, and Joal Zumaya do -- increases the risk that a pitcher will experience shoulder problems.
Also, greats like Ryan, Maddux, and Clemens don't make the Inverted W. Instead, after they break their hands their elbows never go above the leve l of their shoulders.