There is sudden onset, within 1 to 6 hours, of alot of diarrhea, abdominal cramps and nausea or vomiting. The symptoms improve within 8 to 24 hours. There are other forms of food poisoning like E.coli O157 from undercooked meats where the child may become very ill and have severe, bloody diarrhea. These children need immediate medical evaluation and may need hospitalization. Children are contagious with diarrhea caused by viruses for a day or two before the onset of diarrhea, and as long as they have diarrhea.
Hand washing is key to reduce passing it on to others.
Severity of diarrhea varies with the age group. A younger infant will take less diarrhea production to get dehydrated. Remember that breast-fed infants are different and have alot more stool production. Keep in mind that other fluid losses like vomiting and sweating from a fever will add to further fluid loss and increase the risks of dehydration. Severe diarrhea is present if the child is having 7 to 8 or greater stools of large volume that are running down the legs and require a total clothing change.