My argument for treating prisoners better than children (at least in regard to being paid for working) is that once you leave prison, you're likely in a really bad situation unless you have money to help with the transition.

Finding a job is tough as an ex-con. Your support network may well be gone, or it may be entirely composed of people who enabled you in the first place.

Most kids have years of supported, gradual transition from unpaid chores to fully-independent adults, although there are obviously exceptions to that. I feel really badly for kids who enter adulthood from foster care.