This could be a problem as then DD would be required to file a return, right? Currently, she is not required to file a return which means her summer job income did not need to be included in our income for ACA purposes,
Possibly. It would depend on if your DD's income hit any of the thresholds in Chart B (or, less commonly, any of the situations in Chart C) in "Who Must File?" in the instructions for Form 1040. For my kids, that was usually around $12,000 of income, but the rules in Chart B depend on the mix of earned and unearned income.
As for the other replies in this thread, I think I've changed my mind again.
To me, the hierarchy of authority goes:
Constitution
--> Law
----> IRS Publications
------> Popular websites
Out of curiosity, I went and looked up section 529 in the law, and it is surprisingly readable. In relevant part, it seems to say that they money is includable in gross income of the distributee. (Which is the point in the law cited by one of the popular websites listed above by @RetiredAt55.5.)
That makes sense, for all the reasons everyone above has pointed out.
It seems that Pub 970 isn't clear enough on this point and may be making assumptions. I find this disappointing.
@RetiredAt55.5, regarding the instructions for Form 1099-Q, yes, I read that. But that's just telling the 529 custodian how to complete the form; it doesn't say anything about whose taxable income it might be. Also, if it is always taxed to the distributee (as the websites say and the actual law also seems to say), then I don't understand why the IRS would have box 6 on there.