A tan is the body’s defense against sun damage. A dark tan means a more effective defense. But if you have to work at your tan, or burn first, then tan, you do not have a strong defense against skin cancer. We now know that sunscreens do not completely protect you from getting melanoma if you use them to go out in the sun longer. If your children do not easily and quickly darken, you should avoid excess sun.
Read more here: http://www.aocd.org/skin/dermatologic_diseases/sun_protection_for.html