I hate to be repetitive and go back to my previous answer, but it's my sense that including a provision that makes the purchase of sexual services, in general, a crime doesn't really do any good, because it's not narrowly focused enough to get at what we're concerned about.
It's inherent in your question that the concern is not the purchase of sexual services by two consenting adults. The concern is the exploitative relationships that can evolve, and have evolved from these contacts over history.
We should be criminalizing any involvement of children—