I'm with Linjeakel -- whatever will get kids to actually *read*.
And, within some limits, whatever they *will* read.
Reading is one of the most important life-skills there is, and can take a reader in so many directions over a lifetime.
If she wants superheroes & he wants family-stories books, I certainly wouldn't discourage them.
Also, kids' picture-books (or grown-up books w/ lots of illustrations a.k.a. "pictures") just aren't the same when e-books -- somehow it just isn't the same experience as turning real (physical) pages back & forth as you go through the story.