Large Fluid Intake

Many toddlers are especially fond of carrying around a cup with juice or milk all day long, and this can lead to large fluid intake. A toddler urinates about half of the fluid he intakes every day. The average toddler only needs about 2 oz. to 3 oz. of fluid per day per pound of body weight.

Urinary Tract Infection

Frequent urination is only one of the symptoms of a urinary tract infection in a toddler. Parents should also watch for signs of abdominal pain, fever or painful burning while urinating.

Voiding Dysfunction

Worried they're going to miss something interesting, some toddlers do not empty their bladders each time they go to the bathroom. This can cause the toddler to require several short bathroom breaks even though he's not actually urinating that much fluid.

Diabetes

Excessive urination can be a sign of both diabetes mellitus or diabetes insipidus. In cases of diabetes mellitus, children also normally increase their food intake and may suddenly lose weight. An uncommon disorder, diabetes insipidus occurs when there is a lack of anti-diuretic hormone from the pituitary gland. The disorder is normally marked by poor growth and sudden bed wetting. On top of frequent urination, both forms of diabetes also cause increase thirst.

Other Causes

Other less common causes of frequent urination include kidney disease, pollakiuria and vulvovaginitis. Vulvovaginitis only occurs in girls and is usually caused by either poor bathroom hygiene or irritants that have come in contact with the vagina, such as bubble bath. Pollakiuria has no known cause and no other symptoms beside frequent urination. However, pollakiuria normally disappears within three months.

Source:

Keep Kids Healthy: Frequent Urination

Doctor Hull: Frequent Urination

Wrong Diagnosis: Frequent Urination in Children

More Information:

WebMD Children's Health: Urinary Problems and Injuries