In fact, one of his books (that I unfortunately picked up in the library as an adolescent, having liked his Xanth books) features a four-year-old girl who likes to have (explicit) sex with adults, and also a very explicit scene of necrophilia. I have stayed away from him ever since.
Yeah, I tried to direct my dd13 toward his Xanth series when she was about 10. She was done reading HP and LOTR and wanted something else in the general sci-fi genre. She honed in on the Incarnations of Immortality series and, while I admit that it has a good story line to start, I was concerned about the two pieces I recalled from the first book: a suicide and the baby who was a child of incest. I did let her read them after discussing my concerns but she decided that they contained things she didn't want to read herself as she got further into the series.
eta: This isn't exactly sci-fi, but dd13 did move a bit from that genre after her Piers Anthony experiment. She really enjoyed some of Michael Crichton's books such as Jurrasic Park and the Andromeda Strain. I think that she was a bit older when she read those, though (about 12).
