I don't think Scooby-Doo is too scary. If anything, it can be "too campy" at times.
Also, I don't see the adult themes. Personally, I think the girls are the perfect age to be watching the show.
When it comes to Scooby-Doo
, it depends on what era you're talking about. The original series from the late 1960's is pretty tolerable. Anything with Scrappy-Doo is, in my opinion, pretty bad (sorry, but I think Scrappy is rather annoying). I also can't stand A Pup Named Scooby-Doo
(the writing on that show was just horrible). The most recent series, What's New, Scooby-Doo?
is actually pretty decent.