Phenytoin has been used for many years to treat children with partial seizures and generalized tonic-clonic seizures. It is not effective for absence seizures, so children with both tonic-clonic and absence seizures will need to take another seizure medicine as well (combination therapy). Often phenytoin is not chosen as a first-choice seizure medicine for children, however, because of its troublesome effects on appearance (see common side effects). Almost one-half of children who take phenytoin for a prolonged period experience overgrowth of the gums, which is made worse by braces.

Jerking of the eyes and problems with coordination are common when the amount of phenytoin in the blood nears the top limit usually considered effective. Because of the way the body digests this medicine, at certain levels a small increase in the dose given can raise the amount in the blood a great deal. This can make side effects much worse.

To keep side effects at a minimum, the doctor probably will prescribe a low dose to start and increase it slowly. Children usually start with a dose of 5 milligrams (mg) for each kilogram (kg, about 2.2 pounds) of their body weight per day. This is usually given in two or three equal doses. The form of phenytoin in the liquid suspension is absorbed faster than the form in capsules, so giving a large dose only once a day is likely to cause too many side effects.

Most children do best at about 4 to 8 mg/kg per day. The highest dose recommended for young children is 300 mg per day, which can be given using the liquid suspension. Children older than 6 years and teenagers may require the minimum adult dosage of 300 mg per day.

If the liquid is used, it is very important to thoroughly shake the bottle immediately before measuring, because phenytoin does not dissolve well. A child who gets a dose from a bottle that has not just been well shaken may get too much or too little medication.

Phenytoin usually is not given by mouth to infants because they do not absorb it well and are prone to side effects or seizures as a result.