Get your kids a medical power of attorney form, ask them to fill it out, and get it notarized. It's not set in stone, they can change it any time they want, but it's something at least.
Gundersen Lutheran, one of our two local hospital systems, has this boilerplate MPOA which is sufficient for Iowa, Minnesota and Wisconsin:
In addition, the National Right to Life Committee has information on setting one up for whatever state you live in. It would make sense to have one that satisfies the requirements both of your home state and the state in which your college-aged child is living at present.