Doing jobs / working as a kid is perfectly alright if it contributes to their education, teaches them skills for life or helps them learn how to become an independent individual. But within limits. They also need time to grow, have fun and go to school.
Other than that, children will consensually work if the alternative is seeing their little sister starve. Help contribute to the family income or happily skip school if able. Under a certain age, children are regarded as not very wise, unable to consent and easily manipulable. For example by cruel or stupid parents.
That is why it needs to be banned to a certain (and arguable) age. Instead, the state/society needs to provide for poor children, and protect them. Sometimes even from their parents and themselves. Until they’re grown enough to make their own decisions.