Because The noun makes it an identity and a trade the child practised. It also quietly moves the wrongdoing onto the child and away from the adult who paid.

Because Prostitution implies a transaction between parties who can both agree to it. One of these parties is a child, so the word describes a transaction that legally cannot exist.

Because Pornography is a genre adults consent to make. What this material records is an assault, and every copy is a repetition of it. The guidelines now mark the old term red.

Because Tourism is a leisure activity, and attaching it to the crime lends the offender a holiday's worth of innocence.