Children probably are well aware that it exists. What will be difficult for some 9-12 year old children to grasp is why it is unethical. The anonymity of the victims in many cases masks the concrete damage that results. Some 9-12 year olds may be able to understand this, but others may have difficulty with it. For many 9-12 year old children, an idea is not a thing.
Ideas are too abstract.
Then copyright issues should be manageable for the proposed curriculum. Hate speech should be understandable. What will need to be underscored is the cowardice of using the anonymity of the Internet to hurt others feelings. Most 9-12 year olds will be able to relate such acts to slanderous graffiti scrawled anonymously on school walls, for instance. Certainly, high profile cases like Columbine can be used to demonstrate the destructive power of hate speech.