I think, like everything, you start off at the basics and then you can get darker than before... just as long as you don't turn a Batman & Robin... getting parents sending angry letters- etc. those who have the Batman DVDs and have seen the special features- you know the story I'm talking about.
Anyways, Constantine showed hell- don't think many children had problems- just show hell like that... light, but dark. Just don't show people suffering and wailing in pain- but a place surrounded by hell fire with crawling demons and things? That would be "family friendly" (in a way, lol) while still being hell.
Fans yelled "science fiction" to FF, how more science fiction can you get than SS and Galactus? Fans yelling for demonic forms and possibly hell- then the sequel will probably show us that.