First of all, I am sorry to hear that your child has ptosis. I would definitely recommend that you take your child to see a pediatric otolaryngologist
(ENT aka ears, nose, throat) or a pediatric ophthalmologist
to be evaluated. Ptosis in very general terms refers to sagging or drooping of body parts. Most people that talk about ptosis, are referring to drooping eyelids. There are 2 mains muscles that control eyelid opening; the levator muscle, and Muellers muscle. The levator m. is controlled by cranial nerve 3, and Muellers m. is controlled by sympathetic innervation.
There are multiple different things that can cause ptosis of the eye lid (nerve damage, weak muscle, etc) but the common problem is that the lid doesn't stay open like it should. Depending on how much it "droops", it may or may not impact the visual field of the patient. That is why without examining your child thoroughly, I cannot tell you with certainty whether there is visual field obstruction due to ptosis. This is something that will be evaluated at an ophthalmologists visit. I hope this helps.