When children are vomiting frequently, resting the stomach is necessary. Stop all dairy products. For children under a year of age, use an oral rehydration solution such as Pedialyte. For children over a year, you may still try Pedialyte, but some children will refuse it. Older children may be given any clear liquid such as soda pop or fruit juice.
Fruit juices have to be used judiciously because they can make diarrhea worse.
The secret to getting through the vomiting phase of stomach flu is frequent small amounts of fluid. Just start with sips, and advance the amount as tolerated. Changing your child's diet is not necessary if he or she has diarrhea without vomiting. Aside from the removal of too much fruit juice, dietary modification is not going to make much difference in the frequency or duration of diarrhea. If your child is supposed to have diarrhea for 10 days, they will, regardless of what you do dietarily.