I did a search for titles with obamacare children here on FR and the only thing that came up quick is someone "in HR" saying BC&BS are telling everyone that it's a MUST lol and I'm too tired to search any deeper atm. :)
I think the person on that thread was talking from the perspective of the business owner, since they work in HR. So, yes, it is illegal for the business to not offer coverage to the child, even if they are married, have a job, or have their own policy. I don’t think it’s illegal for the policy holder (the parent) to remove the child from their own policy, or opt not to take advantage of the dependent coverage in the first place.