I'm an optometrist in the San Jose area in California. I won't pretend to know all the answers to all of these cases, but I think I can help with some. The symptoms are a bit different so the diagnosis or answers may not all be the same.
The first impression is that the child is making it up or just wants attention. But the children I have seen are intelligent and I felt they really were seeing these spots. And you can tell with all the various people, even older people that they couldn't all be making this up!
1. The most likely answer is migraine aura. (the visual spots and symptoms). Migraines often are genetic and can start at young ages. Some people get only the headaches, some only the visual auras, some both. Constiction of the blood vessels in the retina or back of the eyes (from stress, excitement, physical activity, certain foods) causes these spots. Eveyone sees it differently, it can look like rainbows, blurry vision especially in the periphery and can change and move to the central vision, color spots, geometric patterns, etc. It usually lasts for 10-20 minutes. When the blood supply returns to normal, the spots go away. If the vessels over-dilate near the scalp/head area, then it causes the migraine headache. Triggers can be bright light, like suddenly going outside in bright light without sunglasses, stress or just being busy or active, foods such as MSG (watch out for chinese food), chocolate, beer or alcohol, cheese, and I'm sure processed food may help to trigger which one response suggested. It's easy to find foods that trigger migraines on the internet. You can also learn to control the initial constriction of the vessels with biofeedback instruments which measure temperature of extremeties or fingers to help prevent the headache. In some people, the spots are a sign that the headache is coming.
2. There are small pieces of debris in most people's eyes, or floaters that can give spots, lines, and spiderweb appearance. They are easiest to see looking at a light colored background like a computer screen or blue sky. If you pay attention to them, it can drive you crazy cause you see it all the time. They move around slowly. Best is to just ignore, and usually you won't see them very often. But be sure to get a dilated eye exam to make sure they are the normal ones.
3. One response mentioned seeing the retinal blood vessels. Since the vessels are on top of the photoreceptors, you should see the shadow of the vessels all the time. But the brain suppresses this, unless light is shined in the eye at an angle and the brain momentarily picks up the shadow. Also the white blood cells coursing thru the vessels are large enough to see the shadows of these which look like clear little bubbles or translucent spots. It's usually only when the blood flow changes that you can see this, like suddenly standing up from a sitting position, you see the "stars" for a few seconds.
4. I'm sure there may be possible other explanations for some of you so be sure there is no brain tumor or anything unusual. And if you see the color spots all the time, I just don't have an explanation. But if you read all of these, many have in common the migraine aura symptoms, and many have remarked about headaches or family history.
Hope this helps,
Milpitas Optometric Group