Image via Wikipedia"This could be a sign that your child favors one eye over the other. She may tilt her head because each eye may have a different visual acuity at near or in the distance. Perhaps she sees better and clearer with one of her eyes.
Another cause of your child's head tilt could be the result of a convergence insufficiency or a convergence excess. These conditions may indicate that she is having difficulty teaming both eyes together without experiencing blur or double vision. Her coping strategy could be to suppress the visual information received by one eye, shutting it off, in effect, by tilting her head."
Eye Can Too! Read: Does Your Child Have a Head Tilt?: