Prostitution can and in some cases IS a job that children are involved in. It's not even illegal in some areas, probably.
Granted you usually think of at least semi-coercive "pimps" being involved, which counters the whole job idea, but that's not always the case, I would bet.
Edit: Additionally, children working in anything beyond odd jobs like babysitting, mowing lawns, etc, are more likely to have some amount of pressure on them to do so.
Might be parents, society, or whatever, but the pressure is still there, and that semi-coercive force is in some ways analogous with the situation of prostitutes.
Exceptions of course exist.