Most fevers in children are caused by viral infections and settle on their own. The key is to watch your child rather than just the thermometer. Reassuring signs include staying alert when awake, drinking fluids, passing urine, and perking up somewhat when the fever comes down.
Seek urgent medical care or call your local emergency number if your child has difficulty breathing, a rash that does not fade when pressed with a glass, a stiff neck or dislike of bright light, repeated vomiting, a seizure, becomes very drowsy or hard to wake, has cold or mottled hands and feet, or shows signs of dehydration such as no wet nappies, no tears, or a sunken soft spot in a baby.
Any fever in a baby under 3 months, a fever lasting more than about 5 days, or a child who simply seems much more unwell than a usual cold should prompt a call to a doctor. Trust your instincts; you know your child best.