The only reason I rate this at TV-Y7 is that, with the talk and travel between the two dimensions, it could get confusing for anyone younger. Other than that, I think it is great. I even like to sit and watch it with my kids!
I wouldn't actually recommend this show to under 5's, but my 4-year-old son loves this, as does his 8-year-old sister. My main concerns for the little one are mild violence and that he doesn't really follow the plot all the way through. Still, the goodies always end up defeating evil, and it shows values of team spirit and working together.
A very good sci-fi related show for today's kids. The futuristic technology, in my opinion, keeps kids interested in the show because they can possibly relate to the technology they see in their everyday world.
I have watched all four seasons of Code Lyoko and must say that I have not seen a show that is at all like it. This show has everything from battles that are good but not intense and comedy that just flat out makes you laugh. It is awsome and can be watched by all ages. Some say that it is a baby show because of its lack of violence compared to Teen Titans, but in the end the storyline is better, and the characters have a better attitudes.
When it comes to violence you can't find much of an issue. Most fighting is done against weird virtual monsters, and when they do attack a human they make sure you can't see it (except for in the fourth season sometimes you have to look really close to see it).
When it comes to sexual content, at times the femal characters are seen in bikinis, but this changes by the third season and the don't wear it anymore. There are flirtatious jokes in the show, but they are more kids styled jokes, not teengager style.
Also, there is a little bit of inappropriate behavior to authority, but as time progresses the characters build a relation with their teachers, and they always learn from their mistakes about messing with their teachers. Only once is it considered a joke and that was to avoid another X.A.N.A. attack.
Basically, if your child is at least seven they will be able to enjoy it and will have a marvelous time watching it as the story unfolds and the enemies become stronger and the heroes have to overcome them through teammwork, which seems to get better as time progresses.