Two posts here sound contradictory, but they're not. Here's the explaination:
If you walk up and order a sand-bucket kids' meal, you're going to pay X amount. The sand-bucket is included in the price of the meal -- it does not cost extra. HOWEVER, if you don't want the bucket, you can ask for the meal WITHOUT the bucket, and they'll deduct a dollar or so from the price of your meal (making it quite a bargain by Disney standards).
Kids' meals are also a good option for adults who just had a buffet breakfast or who are planning a big dinner.