one can easily drink 4-6 sodas a day on a cruise... especially if they spend much time on the pool decks. However like PH8 said, tea, lemmonade and occasionally fruit punch are available in the WJ cafe, and are also available in the solarium where the buffet is set up in the evenings...
So if your family really doesn't consume much soda, then you shouldn't need a beverage package. However... if you decide you might want soda, then you could get yourself a beverage package and let the kids use your mug when they want a soda... that way you could purchase one beverage package and share it thus reducing the cost of beverages to make a one beverage package more economical..
Although i don't think RCCL would appreciate you doing this.. it's not as if they really check your sea pass for the bev package... the mug usually denotes that you have a bev package.. and if there was any question your child can always say they are getting it for their parent..
Anyhow... you do have options, and i am sure you can be creative if need be.. I will say though, the beverage packages and wine packages are one way for the cruise line to make money. Since stateroom prices are low, and fuel prices are up they gotta make money somehow right ????