This is an interesting debate. I think cinema has gone full circle, and we are now living in 1975 again, the era when George Lucas made Star Wars as an antidode to the cynical, depressing cinema of the day.
I actually thought that Star Wars III was one of the worst of the series for several reasons, not least of which is that it lost all the fun of the series and Lucas' protestations during I and II that it was meant to be for kiRAB went out the window. Batman Begins is another example of a franchise gone adult, the recent Superman another.
Bond was never all that kid-friendly, but it always provided escapist nonsense fun. Being as we live in such brutal times, I think more escapism is actually what punters are looking for, not more brutality. Look at the success of the critically panned Narnia over the critically lauded and "darker" King Kong last Christmas.
Like I say, it is a really interesting debate but I for one think that when it comes to blockbusters at least, there is an untapped desire for more fun, not less.