For a study, researchers sought to outline the progression of vomiting, diarrhea, fever, and clinical deterioration in children who initially have simple gastroenteritis. An after-hours primary care provider conducted this investigation as a 7-day prospective follow-up study. Generalized linear mixed modeling was used to examine the progression of vomiting, diarrhea, and fever. The potentially more challenging courses for young children (≤ 12 months) and children who had experienced severe vomiting were outlined separately since these groups were more likely to become dehydrated. Additionally, the symptoms displayed by kids who deteriorated throughout follow-up were described, and the day(s) most frequently linked to deterioration. A total of 359 kids visited the after-hours primary care service with simple acute gastroenteritis. Of them, 31 (8.6%) had a severe condition and needed referral or hospitalization. In most children (>90%), all symptoms subsided within 5 days. Vomiting and fever subsided quickly, but diarrhea did so more slowly, especially in infants 6 to 12 months old. The frequency of vomiting was higher at presentation, and the frequency of vomiting and fever was higher during follow-up in children who got worse. The most significant indicator of deterioration seems to be the frequency of vomiting rather than the length of each episode. Explaining the typical symptom length and concentrating on alarm symptoms were crucial when advising parents. Children who vomit more frequently at presentation or follow-up should raise clinicians’ attention since these kids are more likely to get worse.