I've watched the entire series (By that I mean the Japanese edition, the English edition is terribly dubbed). There are a couple of violent parts I could recall in the 1st season such as one of the character's arm being put into a container full of medicinal fluids (in the Japanese edition of course, the English one shouldn't be a problem). In the 2nd season (though I doubt this will be shown in the English dub), there are some horrific scenes of the fate of the city the main characters are living in (Some spoilers here), it turns out mankind is at war with robotic beings, and this isn't a cartoonish war, this is guns and people dying and other whatnot (I have NO IDEA how they're gonna dub this because the 2nd season revolves around this story). The 2nd season revolves around War and "Despair" of the survivors of the future war. With the English dubbed version, it would be suitable for kids (not too young though, you don't want them to be addicted to a card game that becomes a bit too complicated). However, as they watch the English dub, it's likely that new episodes will be released very slowly so they would attempt to watch the Japanese dub on the computer with Subtitles. Basically, I would recommend this show to the older viewing audience (at least 12+). If your child does not have access to internet, I would recommend it to be (8+).
As for my own personal review of this series, I may be a bit old to be watching stuff like Yu Gi Oh, however this is series I've definitely loved. Some lines are a bit cheesy but this show ultimately kept my childhood alive, the only problem is that it's over.
My Ratings:
Creativity - 4/5
Characters - 4.5/5
Story - 5/5
Production - 3.5/5
Lasting Appeal - 5/5
I've watched every episode of this series, although it leaves me wanting more, this is definitely one series I will never forget.