As in an adult, a child's normal body temperature is 98.6°F. If your two-year-old's temperature goes above 100.4°F, then he is considered to have a fever. Keep in mind that forehead and ear temperatures are not very accurate. Mouth, underarm, and rectal temperatures, though varying somewhat from each other, are the most accurate ways of determining your child's temperature and state of wellness.
Fevers do not always have to be treated. If your child's fever is under 101°F and he seems fine — alert, happy, and his typical self — then you can let it run its course. If it lasts more than a day, though, call your pediatrician and describe the symptoms accompanying the fever. Your child may benefit from a fever reducer, such as acetaminophen, ibuprofen, or a combination of the two to keep him comfortable. (Some pediatricians recommend alternating age- and weight-appropriate dosages of each.) Take his temperature every couple of hours when he's sick; let your doctor know if it goes higher than 103°F, if he is shaking or shivering as well as feverish, or if he seems delusional.
Most fevers are short-lived and do not get very high. Even if they do get high at night, they are often lower during the day and run their course within a day. Fevers break suddenly and then temperature returns to normal.