We faced this in 2014 when DH was between jobs. We never actually got the college kid signed up, we had enough problems with getting the rest of us signed up. What we were told, take it with a large grain of salt, was that after we signed up with any dependent kids, then the non-dependent ones had to sign up on their own, and they would be linked to us. I never quite learned how their coverage got linked to ours, but that was the implication. We were covering a short window, a few months and we were back on a new employer plan. The whole "ACA looks at annual income" vs "Medicaid looks at monthly income" meant we were shunted to Medicaid while DH was technically earning nothing, despite annual income appropriate for ACA subsidies.
If/when we attempt to use the ACA for health insurance again, I will find a facilitator to walk us thru the minefield that is NYStateOfHealth. At least in 2014, online you could not make any changes or corrections during the application process, or you were locked up and told to call and go thru a phone rep. Phone reps varied in their understanding of the process, and in their competence. One that was competent tried to fix our application, unlock it so we could continue online, and tried to walk us thru doing it ourselves, but our side of the interface didn't match what she told us should be there. She ultimately had to input info for us as we literally could not get to the screens. Hopefully NY has improved the website, or you get to use the federal one.
Sorry I can't be more helpful.