Pretty much ~everybody on the internet I can find talking about the issue both mischaracterizes and exaggerates the extent of child sex work inside the United States, often to a patently absurd degree. Wikipedia alone reports that there are anywhere from "100,000-1,000,000" child prostitutes in the U.S. There are only ~75 million children in the U.S., so I guess Wikipedia thinks it's possible that more than 1% of people aged 0-17 are prostitutes. As in most cases, these numbers are sourced from "anti sex trafficking" organizations that, as far as I can tell, completely make them up.

Actual child sex workers - the kind that get arrested, because people don't like child prostitution - are mostly children who pass themselves off as adults in order to make money. Part of the confusion comes from the fact that the government classifies any instance of child prostitution as human trafficking, regardless of whether or not there's evidence the child was coerced. Thus, when the Department of Justice reports that federal law enforcement investigated "2,515 instances of suspected human trafficking" from 2008-2010, and that "forty percent involved prostitution of a child or child sexual exploitation", it means that it investigated ~1000 possible cases of child prostitution, not that it found 1000 child sex slaves.

People believe a lot of crazy things, but I am genuinely flabbergasted at how many people find it plausible that there's an entire underworld industry of kidnapping children and selling them to pedophiles in first world countries. I know why the anti sex trafficking orgs sell these stories - they're trying to attract donations, and who is going to call out an "anti sex trafficking" charity? But surely most people realize that it would be very hard for an organized child rape cabal to spread word about their offerings to customers without someone alerting police.