My daughter's skin is very sensitive & also allergic to sunblock. Is it true that you can use desitin (zinc oxide diaper cream) diaper rash cream as a sunblock?

Sunscreen. A good sunscreen that is readily available over the counter and does not cause eczematous reactions is made by vanicream, look for it at your local pharmacy.
Use sunscreen! The fda has made our jobs easier when it comes to sunscreen labels. I would go with a broad spectrum sunscreen that is spf 30+. There are several formulations that you could try to see what she does not develop an allergy to. Consider simple zinc or titanium based products. Any concerns, see your dermatologist who may be able to provide you with samples of sunscreen to try.