Well, to be fair, it IS China. Given China's way-too-free market that's 50% bootlegs, 40% lead-based children's toys, and 10% making our clothes, I think assuming you can legally sell your child there is a completely plausible mistake to make. Heck, given the other human-rights violations China racks up on a regular basis, I'm actually surprised it's not legal to sell your child there.
Yeah it's a tragedy. But let's not blow this out of proportion. The children are victims, but so is the couple. They suffer the rapidly-spreading MMO addiction, and they probably have some deeper commitment issues. And do we even know who bought their children? Could've been potential loving parenting figures. (Which is actually plausible. Especially in the case of the boys. Every parent couple in China wants a boy.) So it's not necessarily the end of the world for those kids.