Why do toddlers get dark puffy circles under their eyes?

Venous congestion.: Toddlers and older children manifest this sign of dark puffy circles under the eyes as a consequence of venous congestion--typically triggered by allergies or by sinus infections. Also known as "allergic shiners", this collection of blood vessels becomes congested with blood backed up because the swelling of the nasal passages blocks the normal progression of blood through the nasal/eyelid area.