I am no parent, but I see nothing wrong. I have read the reviews and disagree with the ones that say "too much undergarments". When they are in undergarments, they are often less reviling than what you would see in a high school in America. As for the violence, I've seen worse on Spongebob. The reason for 6+ rating is it can be complex at times and can get confusing for younger viewers. (although my brother just watches it and pretends to know whats going on)
As for the show itself, aside from having slightly outdated animation in the 3D world of Lyoko, it is a great show about a group of kids who attempt to save a virtual girl. I had watched it when I was young (and when Cartoon Network was a decent channel) and enjoyed it. When I went searching for the name for two months and finally found it I watched it whenever I could. I personally think its great and this french cartoon is better than most of the cartoons in America.