Soft fizzy drinks should be forbidden during diarrhea and vomiting:

Drinking flat soda or other carbonated beverages is harmful when it comes to treating a dehydrated child. It is not only an invalid substitute for specially formulated rehydration drinks (contains less than one tenth of the required salts concentration) but it can also augment the diarrhea and vomiting. It usually increases the diarrhea by means of the large amounts of sugar present in it (7 times what is recommended). It can also increase the vomiting by causing distension of the stomach. Add to this that the large amounts of sugar actually increase on the longer term the thirst sensation of your child.

While it is a classic advice for the older generations to give your child flat fizzy drinks particularly seven-up or sprite whenever he/she has diarrhea and/or vomiting + dehydration. The advice is stronger when they tell you your child will not like the taste of the rehydration solution, but will like seven-up. You should avoid this as much as possible.

The best fluids to be offered for your child in these conditions include breast milk for the infants, oral rehydration solution for all ages (available in pharmacies), water or at least diluted fresh juices. Examples of oral rehydration solutions available include Hydrosafe, Rehydrozinc, Rehydran or Pedialyte.

When your child refuses the oral rehydration solution this is because it is salty; he/she will not like it and drink it unless he/she is really in need of it. Thus, for the anxious mother, stay reassured; if your child refuses to drink the oral rehydration solution then he/she is probably not dehydrated.