Dark circles under the eyes of an infant or toddler are a fairly common symptom of a number of benign conditions, most often allergies or hay fever. Though they can have a somewhat alarming appearance, especially at first, these dark circles -- commonly called "allergic shiners" according to Baby Center -- are a common occurrence and are not particularly dangerous or alarming.
Dark circles are common to people of all ages, but small children and babies are especially susceptible due to their immune system's lack of experience dealing with normal ambient microbes. Sometimes the dark areas are more sensitive to pain. According to "Parents" Magazine, these dark circles are more prominent and noticeable in children with fair skin.
According to About Kids' Health, the most common cause is a congested nose. A stuffed up nose causes the nasal cavity to swell, which, according to Baby Center, restricts the blood flow of surrounding veins, causing them to swell and appear darker. The most common causes of a congested nose in toddlers are allergies and hay fever, though it could also be caused by chronic sinus infections, recurrent colds or blockage of the nose -- which is probably the cause if you notice your child breathing more through his mouth than his nose.
If your toddler has dark circles under his eyes due to allergies, he will likely have other symptoms as well, including wheezing, coughing, sneezing, runny nose and itchy eyes.
It can be very alarming when a child's face suddenly goes from rosy-pink to blue-and-black. Parents often fear that their child is not getting enough sleep, is getting poor nutrition or, worst of all, the victim of some kind of abuse as a result of seeing the allergic shiners under their eyes. Fortunately, they are nearly always the result of the comparatively less-alarming nasal swelling.
The best way to treat the dark circles under your child's eyes is to treat the cause: allergies. Keep a particularly close eye on your child for a few days, paying special attention to potential allergy sources. Ask your pediatrician what kinds of treatments are available for childhood allergies, and consult your nearest pharmacy. Nasal sprays and antihistamines can help relieve congestion.