Parents often think to apply sunscreens to exposed areas of skin, such as the face, but feel safe about areas under the clothes. Ultraviolet radiation, however, can penetrate clothes. Prolonged exposure can damage the skin. Typical cotton T-shirts offers sun protection equal to only SPF 7.
One solution for long outdoor days: sunscreen under the clothes.
A less messy solution: sun protective clothing.
Sun protection depends on the fiber, weave, and fabric color. Sometimes a substance that blocks UV radiation is added to the fabric. The ultraviolet protection factor (UPF) of fabrics is similar to the SPF of sunscreens. A high UPF (40 to 50) is nice, because the ongoing sun protection is affected by stretching, shrinking, wetness, laundering, and the normal wear and tear of childhood.