Most vomiting and diarrhea is caused by a viral infection or minor reaction to a food that disagrees with your child. Vomiting is forceful throwing up and differs from effortless spitting up which is common in infants. Diarrhea is stool that is watery and occurs more frequently than normal. Most stomach viruses cause vomiting for 6 to 24 hours, followed by 4 to 5 days of diarrhea, though it can take 2 weeks for a child's bowel movements to be completely back to normal. Symptoms often include decreased appetite, fever and fussiness as well.
Signs that a child is dehydrated include no urine in 8 to 12 hours, dry tongue, and no tears when crying (if over 4 months old). It generally takes more than 24 hours of vomiting and diarrhea for these symptoms to develop.