Look for children's books that have been translated into German.
You should rather look for some German authors that have been translated to the English language; the list should be extraordinarily long and cover a couple of centuries. I think it also makes more sense to read native authors - translations always lose a lot of the quality of the original.
Try reading Erich Kästner, for example "Pünktchen und Anton" or "Emil und die Detektive". He wrote some of the most famous children books.
I would not start with the Brothers Grimm. Although their collection of fairy tales is legendary, their language will be too advanced for a beginner. Also don't start with the classics Goethe, Lessing or Schiller - their language will kill you and you probably won't understand a single word of what they're saying. It's like reading Shakespeare or Joyce's Ulysses in first grade. Not a good idea.
If you like comics, the German translations of the French "Asterix" comics are awesome, but probably already too advanced in their use of language. But that will be the problem with all comics; the German "Werner" series is as (North-)German as it gets, but almost exclusively uses slang that will be impossible to understand for non-Germans. And the problem with comics in general is that their language is always very colloquial and contemporary and lives inside a specific local context. So I'm not sure if comics are really a good start. They're the most fun, though, and I remember reading some French Asterix comics in high school. Asterix is still my all-time favorite, in ANY language; just like Harry Potter, they even have Latin translations.
Talking about Harry Potter: You might be tempted to try this. After all, this is what all the kids around the world read in the English original no matter what their own native language is.
I must say that I found Harry Potter rather challenging: It's BRITISH English, VERY local British English. It's a "stag" and not a "deer", and when English is not your local language, the many differences between American and British are quite confusing.
I've read Potter in English, but gave the German translations to my Mom as a Christmas gift. The translations are well done, BUT they also translated the names of places, animals, creatures and what-not. Just as it was done in the German translations of The Lord Of The Rings. So when you're used to Harry having a butter beer in Hogsmeade, you're in for a little surprise because there is no village named Hogsmeade in the German version.
What you can - and should - also do: Watch German movies. Yes, we do make movies over here, and some of you might even know that Roland Emmerich (2012, The Day After Tomorrow, Independence Day, "Das Arche Noah Prinzip") and Wolfgang Petersen ("Das Boot", Troy, Outbreak, In the line of fire) are Germans.
Watch "Das Boot" in the German original with English subtitles turned on. So you can hear the language but still be able to follow the dialog.
Other great German movies include:
"Im Juli" by Fatih Akin (yes, a Turkish guy who grew up in Germany)
"Die Katze" by Dominik Graf
"M - Eine Stadt sucht den Mörder" by Fritz Lang
Anyway, gotta go. Work's calling.
