Susan,
I thought the same exact thing when I saw the "payment authorization form" in our cruise documents. You have to list out the names of the people who are authorized to charge to your card, but then if you look at the last few lines, it states that all minors have charging privileges and that you have to stop by guest services to change that. So why exactly are we filling out the names in our party who can charge? That one has me a bit confused.
I'm going to take the suggestion of a previous poster and give each of my children "cash". I'll keep it in an envelope in the safe, but will deduct any spending that they do on the ship. My two oldest are 18 and 14 though, so they should be able to keep track of what they spent. My 14 year old has already asked me for a breakdown of what's included and what he needs to pay for. I told him that I would purchase the soda package and internet access for him, but any smoothies that he purchases in the teen room and video game cards are his responsibility. We're going to Disney for 2 days after the cruise, so that will be a little incentive for them to not spend all their money. I know they want to buy souveniers at the parks too. It will be interesting to see how they budget themselves.