I would assume when a woman lends her credit card to her husband and the husband pays the interest of the credit card (or vice versa), it is permitted because it is one entity. Would the same be when I use my father’s credit card and I pay the interest since my father gives me some financial monthly support. (Or maybe I have this all wrong).
The backround of this Halacha is that if a person lends money on his credit card, from a gentile, he has no problem paying the interest, as we may pay interest to a goy. However if he lets someone else use his credit card, and when it will accrue interest, it is considered as if the owner of the credit card has lent money to the other person Jew, and when he repays him, he is now taking ribis from him, which is for bidden.
If a wife uses her husband credit card, they are like one account and one person, ( unless they have a different arrangement) therefore it wouldn’t be an issue, as you wrote, however when a child (unmarried) does it, it will depend on the way his father supports him. If the father pays his bills, then his spending is really like his father’s, and it isn’t like borrowing from his father, however if his father gives him a certain amount of money per month to do with as he pleases, and the bill is now his responsibility to pay then it is would be ribis, unless they make a heter iska between them.
If the child is married, then even if his father helps support him, however he is still an independent person regarding this, and it would be an issue unless they make a heter iska.
Y:D 168-1 and 17, Mishpat Haribis 6-1, R’ Y. Cahen shlit”a.