Or you can read a lot of the older fantasy, for that matter: George MacDonald, Hope Mirrlees, Lord Dunsany, William Morris, David Lindsay, Mervyn Peake, H. Rider Haggard, A. Merritt, etc.
Or not necessarily "older" in the sense of books, but writers who began before the current fantasy craze set in, such as L. Sprague de Camp (alone or in conjunction with Fletcher Pratt, who was a professional historian -- humorous books, but often with some serious points to make as well), Poul Anderson, Andre Norton (though I'm not sure how she'd respond to a series set in the Witch World, albeit these were accepted and loved by generations of sff readers), Katherine Kurtz, Evangeline Walton, Roger Zelazny, Avram Davidson...
And, of course, you also have writers that do come from the YA field, though they didn't necessarily always stay there (personal favorites of mine being Alan Garner and Lloyd Alexander, for instance).
There have also been quite a list of retellings of classic fairy tales as modern fantasy novels, many of which are quite good; those would be worth seeking out, as well.
And as for science fiction... well, you could do a lot worse than reading a lot of the classic sf (I'd suggest the New Wave, but some of that got into rather "adult" subject matter at times, and I doubt that would meet with parental approval, either....). Or many of the Hugo and Nebula winners (not to mention the anthologies which collect together the shorter tales (short stories, novellas, novelettes) which have won these awards, or the Science Fiction Hall of Fame volumes One and 2A and 2B:
Hugo Award for Best Novel - Wikipedia, the free encyclopedia
Nebula Award for Best Novel - Wikipedia, the free encyclopedia
Basically by the time you whittle through even a small portion of all this, you'll be well into your 30s, and if you're still interested in reading those that don't currently meet parental approval... well, you may well be the parent
objecting by that point...