Parents need not fear that and they should be overjoyed that their child has chosen to read as opposed to watching the dreck that is on television or getting engrossed in those infernal video games.
I have always encouraged my children to read. I've never snatched books away from them or lectured them on their choice of reading material.
When I was a child, I would read comic books, MAD Magazine and science fiction that many parents at the time forbade their children to have. I was allowed to read them to my heart's content and eventually I became bored with them. Eventually, I started reading real literature like Jack London, Mark Twain and Charles Dickens, not because my school teachers assigned it to me but because I had developed a love of reading and appreciated good writing. Yes, I also read some sexually explicit novels and even Playboy magazines as I reached puberty. But because it was not "forbidden fruit", it didn't have much appeal and I grew out of it quickly.
Do your children attend public school?