This is one of those instances where Wikipedia is really the best place to start. There is a fantastic Wiki site that covers all of your questions:
The Great Barrier Reef was designated as a UNESCO World Heritage Site in 1981 As such, its geology, ecology and other aspects are presented in detail on the UNESCO site as well:
There are many articles about the Great Barrier Reef offered over the years through the magazine Scientific American. You can search through the list of articles on their website by searching for "Great Barrier Reef":
I find these Sci Am articles to be a good teaching resource.
You don't say what age kids, but if they are elementary school age, I highly recommend the National Geographic site for kids - they have an interactive "online adventure" for the Great Barrier Reef, as well as links to websites and articles for the kids.nationalgeographic
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