Child prostitution is the lowest priority of the FBI's Crimes Against Children Unit (CACU) - 2009 Office of the Inspector General Audit Report.

Explanation of FBI Priorities Ranking:

The first eight priorities are listed in order of precedence.
The final two are all-encompassing functions that support the Bureau’s mission and objectives.
The ten priorities are:

1. Protect the U.S. from terrorist attack;
2. Protect the U.S. against foreign intelligence operations and espionage;
3. Protect the U.S. against cyber-based attacks and high-technology crimes;
4. Combat public corruption at all levels;
5. Protect civil rights;
6. Combat transnational and national criminal organizations and enterprises;
7. Combat major white-collar crime;
8. Combat significant violent crime;

9 Support federal, state, local, and international partners; 10 Upgrade technology to successfully perform the FBI's mission. – https://oig.justice.gov/reports/2014/a1410.pdf

The FBI give much higher priority to investigating CP and abductions, than child prostitution because prostitution is classified as "significant violent crime."

According to CACU officials, the FBI’s mission and resource limitations cause child prostitution to rank as a lower priority. The FBI categorizes non-cyber crimes against children as violent crimes, which currently rank eighth on the FBI’s overall priority list. For crimes against children specifically, child abductions are given a higher priority than child prostitution related crimes.120

Endnote 120: A Supervisory Special Agent assigned to the CACU at the FBI headquarters expressed the opinion that a prostituted child should also be elevated to the status of a “child in imminent danger” and receive immediate attention. [obviously a dissenting opinion] – https://oig.justice.gov/reports/FBI/a0908/chapter4.htm