"Slave" like any word has contextual meaning. In this context I'm using it to refer to the workers who find themselves caught in a coercive political-economic system. Other similar words are wage slave, proletariat, or just working class. The point is that there is an involuntary aspect which likens it to slavery in the more narrow sense. (The narrow meaning of slave I have in mind being "someone forced into labor without pay".)
All that said, in the U.S. there are still slaves as defined narrowly as people who are forced to work without pay. Slavery is used in prison systems, for example, and is not uncommon among human trafficking victims and immigrants (e.g. read Tomatoland). If your children are women, indigenous, black, are born or become disabled, or belong to various other minority statuses they are at even greater risk of getting swallowed into those forms of "literal" slavery as well.