You will need to set budgets for your children. If they each have their own card, they can have a field day without thinking about it. You can check your account during the cruise to see what the spending activity is and take corrective measures if they go over the budget.
When my son was younger, he loved virgin pina coladas and could drink them all day long. It can get out of hand fast if you don't monitor it.
Kids will spend money on drinks (consider soda cards) in the stores, in the video arcades and when on shore. All of their food, entertainment, activities on board ship and kid's programs are covered in the cost of the cruise passage.
I am willing to bet that whatever budget you set for them, they can find a way to spend it! VBG. They will have the time of their life.