Again, it's always best, if time allows, to call the doctor first before making that ER trip. Probably the most common would be trauma, especially instances in which X-rays or suturing of a laceration might be needed. Another common example would be head injury, especially if there was any degree of unconsciousness. Fortunately, most head injuries are minor and don't need hospital evaluation. So if the child is alert and is just upset with a big bump on the head, it would be best to call the doctor before heading off to the hospital.
Breathing difficulties comprise another reason.
If a child appears to be breathing rapidly, is short of breath, or is struggling to take in air, an ER visit would be the correct action. Another reason would be to evaluate and treat possible dehydration. Signs that would make one suspicious of dehydration include a marked decrease in urination, dry lips and mouth, sunken eyes, and a significant decrease in the child's activity level. Another gastrointestinal issue which might warrant an ER trip is abdominal pain. This is a common symptom in children which usually has no serious basis but occasionally needs hospital evaluation.