My daughter is missing enamel on part of her front teeth. What causes this?

Several. Could be caused by dietary habit, trauma ( mechanical trauma or could be caused by some corrosive agents- acidic beverages, emesis), excess Fluoride consumption, infection or hereditary problem could be the cause. Bring her to a dentist and discuss more about dental health and what to do.
Many causes. Loss of enamel could be due to baby bottle caries if the child had gone to sleep with an acidic substance such as milk or a fruit juice that was allowed to pull on the child's tooth. It could also still be enamel but ds colored due to antibiotic staining when the teeth where being formed. Loss of enamel could be due to a systemic condition with other symptoms . You must see your dentist for reason.
Lots of reasons. Could be caries (cavity), damage from trauma, developmental defect in that one tooth, genetic (inherited), or it could be due to one of many systemic problems. You really need to have your dentist examine your daughter now to determine both the cause and the appropriate treatment.