Our cruise had water, iced tea, lemonade, fruit punch (that really sweet stuff), and milk all available at the beverage station (coffee, too, but the kids didn't drink much of that). (edited) Hot Chocolate in those little packets and teabags (and hot water for both) as well.
When we were going out, we brought insulated water bottles and filled them with cold water from the beverage station.
Pop is free in the sit down dining rooms.
If your kids are real pop drinkers, you can also bring your own on board in your daybag.