First of all, the humor and other literary goodies buried in it are really only appreciable by those who have experienced both the childhood worldview and the adult (probably 11 years plus) worldview. When Nathaniel points out the stupidity of a rule, only an adult would be able to recognize the rule and realize that he has misunderstood or is misapplying it. While I enjoyed Calvin And Hobbes as a kid, I enjoyed it much more as I grew up (maybe I missed my childhood).
Second of all, I know some parents might take issue with the way parents (and authority figures in general) are portrayed in the story. They are usually very silly. They may also take issue with the level of violence, though it is really no more violent than the average fantasies I remember creating with my friends on the playground when I was six. I don't have a problem with it, but because some people might, I'm not presenting it as a kids' book.
Third of all, while the Nathaniel Series features only the young Nate, The Spider, The Witch, And The Spaceship is really about grown-up Nate who has to solve the mystery of what is happening to him by searching through his childhood memories for clues. To contrast the world then from the world now, he often complains that the world is more confusing now, including once mentioning his surprise at the existence of sex. I promise there is nothing in there I wouldn’t want a twelve-year-old reading, but for the sake of disclosure so you can make better purchasing decisions, I thought I would mention it. Also, out of 85,446 words, the character of Derek once uses the word “bastards” and once uses the word “shit” – though to be fair, he was being chased by a giant spider at the time so it seems perfectly appropriate to me. In the Nathaniel Series, the strongest language ever used is "Grrrrrrr!" “Shut up!” “Ugh!” and “It sucks!”
I have also had a couple people claim that the scientific jargon might be too advanced for most adults, let alone most children. I very much disagree with this. More than half of the science is made up anyway, so people shouldn’t understand it. I put nothing into my books any harder to understand than is found in Doctor Who, Star Trek, Stargate, or The Foundation Series. I had no problem enjoying Star Trek and Doctor Who from a very young age, so I see no reason why young children today couldn’t enjoy my books.
In conclusion, my books are not for children, but I certainly welcome any children that want to read them (with parental guidance).