Is it normal for my child to have the ability to blur and double vision their sight on command?

Please see your physician for a vision screening test. Very often at this age, there is a change in visual acuity. He may need to see an Optometrist for glasses. If there is a headache, vomiting or nausea, a Neurology evaluation is indicated.

Actually, yes. We can all defocus and blur the vision or change focal point and see double in the unfocused area. As long as your child's vision is normal the rest of the time, this is nothing to worry about. For peace of mind, you can take your child to an eye MD for a basic exam.

Yes. It's not uncommon to be able to voluntarily "defocus" or dealign the eyes to see "double". I absolutely wouldn't encourage this activity and if it ever becomes involuntary, you should seek evaluation from an ophthalmologist