an old car my parents let me take to college for a year after my mission, and I stayed on their health insurance until I was 26 (I think).

I often tell myself that I put myself through college without help, but the car and insurance was probably more helpful than I realized at the time. There was no ongoing to cost to my parents for these things (the car was old and had long been paid for and I paid for gas and maintenance, and I think they didn't pay anything incrementally per month for me to stay on their health insurance because they still had a bunch of other dependents on their policy). But if I would have had to pay for those things it would have cost me more.

We have 529 accounts for all of our kids and have been contributing since they were born. It should be enough to pay for most of their college at BYU or a similarly priced school. But if they want to go somewhere more expensive, that would tap into funds we're planning on using for other things. I'd be inclined to tell them something like the OP is considering to let them know we can help them with that to some extent but that the majority of those costs above what we have in the 529 plan will be up to them.