I was allowed to watch this when I was eight. My parents know this was a mistake, but everything flew over our heads. If you want to watch this with a six year old, go ahead. Just make sure they know its a one time thing (until they're 12 or something). Also, it's probably best if you watch it 1st AND with them. My parents made that mistake and because of it I taught "Does Your Mother Know" to my friends. The movie is full of sex, social drinking, smoking, references to weed, sex, one scene with someone's bare butt (not sexually, though), sex, sex, and sex.
Quality wise, Mamma Mia is a pretty good movie. In movie-musicals, casting directors think that they need to cast big stars who don't always have much (or any) singing ability. So it's no surprise that people like Pierce Bronson sound really bad. But people like Meryl Streep, Christine Baranski, Julie Walters and Amanda Seyfried have amazing voices. Sure, it's not well organized, some singers suck, and it's cheesy (good cheese, but still). That's why I took a star off. But with all it's flaws, Mamma Mia is a very fun movie that tweens and up can watch again and again.