Even with legal constraints companies continue to try to work children. In other countries they do work children (and 70 hours a week and locked in the building) despite access to the same technology which supposedly "freed" the children.
One of the biggest problems at Foxxconn (Apple Iphones) is labor turnover because of crippled and permanently disabled children from exposure to toxic chemicals.
Programming can be crippling if you start it too young. And on my last project (SAP conversion), we had three people die on the project (and a possible 4th who was hauled away but they were a contractor so we never heard about them). One 43 year old just said she felt bad, laid down, and that was it.
We also had 4 heart attacks and 3 cancers. I saw young people walking around with black eyes from lack of sleep. Programming without constraints is horrible and will kill you. Humans are not meant to consistently work 70-80 hours per week for months.
If child labor laws were and are unnecessary then why do we continue to have violations (lots of them) today. Both for excessive work hours and for hazardous violations (including fatalities).
If child labor laws are unnecessary, then it wouldn't matter because businesses would never come close to violating them.
Most people can be evil with very little incentive (Summers experiment, the french electricution torture television show). Unconstrained capitalism gives people a very HIGH incentive. People will behave evilly unless you put in clear boundaries.
It's the difference between sports and war. And we even have rules of behavior for wars. Without them, you end up with genocide and people eating the opposite side's hearts.