Welcome to Cruisemates!
The major cruise lines do sell a soda card. It's good for fountain sodas (not cans). It's not required. If you and your children only drink one or two soda's a day it's probably not worth the price. Canned sodas are generally priced at $1.50 per can.
If you and your children drink soda's all day it would probably be worth the price.
If you'll tell us which ship you're sailing we can give you more specific information.