I'll second (third?) Rush, and Jethro Tull (they do have some mature subject matter*, but little to no profanity, and a lot of it will go over kids heads unless they're fairly knowledgeable and paying attention.
Some of Black Sabbath's early stuff is actually fairly child-safe (if your child is over 8), particularly Paranoid, and [i]Master of Reality[i]. Pink Floyd has an s-bomb here or there, but otherwise everything by Pink Floyd up to, at least, Wish You Were Here, and then everything from Momentary Lapse of Reason on should be fine.
If you count Electric Light Orchestra as Prog Rock, everything they did should be okay (though the opening for Fire On High creeped the crap out of me until I was 14.
*Aqualung is about a pedophile homeless man, Cross-Eyed Mary is about an underage schoolgirl who prostitutes herself. Oh, and there's all the songs about objectivism in Rush albums - and then there's Passage to Bangkok.