Few google search, I see some explanation too, here:

In many animals, including dogs, cats and deer, the retina has a special reflective layer called the tapetum lucidum that acts almost like a mirror at the backs of their eyes.
Humans don't have this tapetum lucidum layer in their retinas. If you shine a flashlight in a person's eyes at night, you don't see any sort of reflection. The flash on a camera is bright enough, however, to cause a reflection off of the retina -- what you see is the red color from the blood vessels nourishing the eye.
If you use a flash in a dark environment, you often get a red eye effect. This is because the light of the flash is reflecting from the retina, which is covered with tiny blood vessels.

The more open the pupils are, the more red eye effect you get in your photos. Red eye is more pronounced in people with light eye color. It is also more pronounced in people with blond or light-red hair and in children.