What causes read dilated vessels in a child's eyes, looking like pink eye?

Need eye exam: When the eye is traumatized from inflammation, infection or even tumors the response is usually a red eye. Eyes look red because of dilated blood vessels. New blood vessels don't form but the existing ones dilate in response to injury. The only way to know what is causing the redness is to take a good history and have a thorough eye exam to exclude things like uveitis (inflammation) or infection.

Lots of choices: Assuming it's not actually an infection, the next most likely would be an irritating foreign body or allergies. Could be a variety of other conditions, some of which are serious, so if it persists, a real exam would be the way to go.