It's not (necessarily). The idea comes from the industrial revolution, in which working conditions were awful and workers were exploited to extremes, coupled with the notion of mass public education. Children then would go to school and not work so that they can have a higher standing when grown up. Depending on the job, the state of educational institutions and social climbing opportunities, this doesn't always apply.

A carpenter, a baker, a mechanic, a welder, would do right in teaching their kids the trade early on, and even have them help out in true productivity. A business owner will bring his teen kid to the office so he can begin learning the family business. That is all good.

A poor kid might spend his Saturdays or weekday afternoons begging for coins or wiping windshields to help bring some bread to the table. That is not ideal, but it is honorable and not immoral. Children have been useful to their parents for most of human history.