Yikes, now I am nervous!
Yes, it is just a gaze problem, no drooping lids etc. The technical term for what the child has is, I think, Partially Accommodative Esotropia. The parents did not even mention it at first and It is just barely noticeable, but of course, since I was scrutinizing the child's face so closely I noticed it. When I (very tactfully) wondered if the child's gaze was slightly misaligned then they told me about it. And yes, I am doing the portrait in 3/4 pose with the affected eye partially in shadow. So hopefully, my subtle "corrective surgery" will not really be very noticeable. Thank you Chris and Tom for your sage words of advice! I'll keep you posted.