Run, do not walk to a pediatric ophthalmologist, or the nearest emergency room. Pupil dilation in only one eye can mean a brain bleed, serious head injury or even a vision problem. Do not sit on this one!!!! I was an ophthalmic tech for 10+ years. If you shine a bright light in that eye and it doesn't constrict, you have a problem. Either optical or neurological, either way, do not hesitate one more second to get help.