What do you do for a five year old with a stumach virus?

Take them to the doctor.

Gastroenteritis
(Viral Gastroenteritis, "Stomach Flu")

What causes viral gastroenteritis?
Many different viruses can cause gastroenteritis, including rotaviruses, noroviruses, adenoviruses, type 40 or 41, sapoviruses, and astroviruses. Viral gastroenteritis is not caused by bacteria (such as Salmonella or Escherichia coli) or parasites (such as Giardia), or by medications or other medical conditions, although the symptoms may be similar. Your doctor can determine if the diarrhea is caused by a virus or by something else.

Is viral gastroenteritis a serious illness?

For most people, it is not. People who get viral gastroenteritis almost always recover completely without any long-term problems. Gastroenteritis is a serious illness, however, for persons who are unable to drink enough fluids to replace what they lose through vomiting or diarrhea. Infants, young children, and persons who are unable to care for themselves, such as the disabled or elderly, are at risk for dehydration from loss of fluids. Immune compromised persons are at risk for dehydration because they may get a more serious illness, with greater vomiting or diarrhea. They may need to be hospitalized for treatment to correct or prevent dehydration.

Children with stomach flu present special concerns. Because of their smaller body size, infants and children are at greater risk of dehydration from diarrhea and vomiting. Oral re hydration solutions such as Pedialyte® can replace lost fluids, minerals, and salts.

Parents of children with severe diarrhea should start oral rehydration and take their child for medical evaluation. In severe cases requiring a visit to the emergency room or hospitalization, stomach flu treatment may involve replacing body fluids directly through the veins using an intravenous (IV) line.