Don't skip the sunscreen. Kids need at least a half-ounce of SPF 15 — or ideally higher — with each application. You need to cover the parts of her body that are covered with clothing too, and give the lotion at least thirty minutes to be absorbed by the skin before heading outside.
Did she get wet? Reapply, reapply, reapply. Also consider hats and clothing with thick materials — even swim shirts with built-in SPF. And limit her time in the sun, preferably sticking to mornings and early evenings.