Where in the Bible does it say child molestation is wrong?
The Bible nowhere speaks specifically of child molestation. Instead both the Law of Moses and the New Testament deal with sexuality in broader terms. The law of the Jews condemns various incestuous relationships, which would include child molestation by a parent. (Leviticus 18) It also deals with rape (Deuteronomy 22), homosexuality (Leviticus 18:22), adultery (Leviticus 18:20), and sexual relations outside marriage (Deuteronomy 22). All those topics could include improper sexual relations with a child or adult.
In the New Testament we find similar passages. Romans 1 talks against sexual sin 'against nature'. He also talks about adultery, and an implied condemnation of other sins including child molestation. The writer of Hebrews says the marriage bed should be undefiled (Hebrews 13:4), which would include this as well as adultery and fornication. In Ephesians (6:4) and Colossians (3:21) Paul addresses parent-child relationships, which would also rule out incest.
The Bible is not a book of rules for each possible situation, but is a guideline for living in general. The summation of the Law and prophets, according to the rabbis including Jesus, is "love God with all your heart, and soul, and mind" and "love your neighbor as yourself." If we constantly think of the best for God and others we won't be involved in such harmful acts as child molestation.