I was playing Diablo when I was 5 (back then it was rated M and considered very violent), so I don't see why minecraft is bad... It's a lot more productive for children (too often our creative juices are squashed by the education system). As for other people on servers being explicit, I guess that's a risk, but at least in my family we were taught the facts of life fairly early. Did I understand it when my mom told my brother and I (I was 4 and he was 7)? Not really, but I didn't turn into some kid who was obsessed with sex.
I mean, they're going to be exposed to these things eventually. As long as their parents do their job of making sure that they understand healthy behavior, that they shouldn't give away detailed personal information and to make sure that any uncouth behavior they pick up online stays online, I don't see how it should be an issue.
And from the experience of being an 8-10 year old on internet games amidst guys who were 16-25, I can say I've had many more positive encounters than I have had bad ones. More often than not I was treated like the little sister of the group.
