Over-the-counter pain relievers like acetaminophen, ibuprofen, and aspirin can be very effective for headaches in adults. Most of these are safe for children, but due to the risk of Reye's syndrome, you should not give aspirin to a child or teenager unless specifically directed by a doctor.
Reye's syndrome is a rare disorder that causes brain and liver damage in children. Taking aspirin while suffering from an upper respiratory infection or chicken pox greatly increases the risk of Reye's. Both of these diseases can cause headaches, which is why it is important to not use aspirin to treat a child's headache.
The symptoms of Reye's syndrome come on quickly, usually over the course of several hours. They start with vomiting, followed by irritability or aggressiveness, and then confusion, lethargy, seizures, and coma. There is no treatment for Reye's syndrome. It is rarely fatal, but it can cause varying degrees of permanent brain damage.
The best rule of thumb is to only use Tylenol when attempting to treat a child for fever or headache. Be sure to only use the recommended amount because too much Tylenol can damage the liver. If the pain or fever is not diminished by Tylenol, see a doctor.