Your Meal coupons are actually in the Disney Reservation System. Your Key to the world cards access these coupons. Therefore every card you have will be able to acess the coupons. If you went into the resturant with 10 credits left. 5 min before you ring out your evil friend buys himself and 9 strangers Dinners you would be paying cash.

I do not know if you can take away access to the meal plan from others, I didn't ask. Every receipt will have how many credits used and how many are left. So if you forget your next receipt will remind you. And it will unless changed in the future total credits and will not seperate child and adult.