Is it? I would think it is in the developed world, where child labor laws are most strongly enforced. It certainly is with runaways (to the profit of "anti-traffick" ngo's). The few (but excessively media covered) cases of middle class (ie, not poor or desperate) girls who have been caught doing sexual favors or sending self-KP to adult guys, certainly seem to fit this pattern.
I remain doubtful with other countries. In countries where child labor laws are less strict or less enforced? There it does seem to be a choice triggered because it's either better paid, or less physically demanding or unsafe, or easily available to girls when other jobs are only or mainly available to boys. Again, anti ngo's show the thread: they rescue children from prostitution to have them
But I'd be interested in knowing more.
Not the first time when stated goals of the War on Children contradict with each other and even undermine each other.