I saw the video about a year ago, the one posted by gamma on this thread and the general summary I got from the video is that:

1. For the immense time and expertise these children are putting into the game, they get a very small cut. And their social life and education will probably suffer so hey!
2. A job comes with security and rights. These children get no rights as they are not on contract.
3. There are predators on Roblox which the company is overlooking (as they are making huge games there, like the Sonic the Hedgehog Roblox game).

It's a very grey area that isn't regulated well.