All skin types care prone to sun damage, thus skin cancer. Therefore, everybody, despite skin color, should apply a lot of sunscreen with SPF of at least 45 to all sun-exposed skin surfaces before outdoors activities, even in winter! That applies especially to children, because the most sun damage happens before age 18 years.
Your skin color, depending on how dark it is, does afford you some protection from the sun (potentially up to SPF 15). Regardless, your friend should still apply sunscreen to her children as the SPF-equivalent affects of darker skin don't make up for applying sunscreen. Sunscreen is always a good idea, regardless of skin-color, and especially when your children are in the sun/swimming during the heat of the day.