Children are afforded more lenience in sane societies (before the law and in social contexts) because they are still developing and not as well socialized/experienced as adults. I assume most pro-piracy people support personal use and not commercial use of content.
The issue is that child labor laws encourage children to pursue cybercrime if they want to make money since legitimate companies will not hire them. This results in a lot of incentive for children to commit cybercrime such as piracy and without the disincentive of punishment they are free to do it. These 2 things are incentivizing antisocial behavior in society.