Almost everybody I know who had more than one child and a family plan kept their employed kids on their plan until they were 26 (if the plan covered the area where the kids lived). Once they were down to one kid, since the others had aged out, if that kid was offered insurance with their job, then they kicked them out so they could stop their family plan and just do employee + one.
I don’t see any purpose to having two plans (theirs and yours). Why have the kid pay any premium if they don’t have to?
Also, I completely disagree with not having young people get vision and dental plans. My daughter has both plans from her employer (has since her first job out of college) and she has used both benefits quite a bit (wears glasses, TMJ, had her wisdom teeth out, etc.) These premiums for one person are typically very low and are worth it.