When I heard about the "too grown up" debacle online (mostly by the media, though) I remember the same thing happening with Harry Potter. Just because it's targeted to children doesn't mean it can't be scary, or complex.
To be honest, anything related to River is sometimes to complex for me, but I get the important stuff, and I'm sure kids do too. And when they watch these episodes, once they are older, they'll find another layer they'll enjoy.