I can only tell you what I did for my teen sons. They had their own soda card, that they could use as often as they wanted, but I told them that the Sign and Sail card had to be authorized by me. If they needed something, which didn't happened very often, they would have to come to me first, tell me what they needed, and then buy.
It is a misconception that you have to have money on a cruise ship when your a teen. They eat when they want, and as much as they want for free. All the teen activities are free. Even if they don't have a soda card, there is plenty of things to drink on the ship, that is free.