The $500 “child tax credit” is tied to income, pretty low income, so don’t count on your college student qualifying unless they hardly work at all. In addition, unless the law is amended, as it currently reads, the income threshhold is zero. Even if they fix it, the earnings limit will most likely just be a few thousand dollars.
https://www.marketwatch.com/story/have-kids-5-ways-the-new-tax-law-affects-you-2018-02-21
Also, the $500 is nonrefundable.