Parents should know that there has been a big shift recently in the science on sun exposure. In fact, many researchers now believe that sun exposure may actually lower overall cancer risk. (I've added some links at the bottom of this message.)
The issue is a bit confusing because UV exposure does indeed cause some cancers, but only skin cancer, and nearly always the least dangerous types (the cancers from sun exposure are nearly always just a cosmetic issue). But sun exposure seems to reduce the occurance of cancer, and increase survival rates, for nearly every other type of cancer studied. Vitamin D from UV exposure seems to be critical to our immune systems, and not just for fighting cancer.
I first researched this when our pediatrician told us to give my daughter vitamin D suppliments, since she was exclusively breastfed for the first 6 months. When I searched through scientific papers not only found that most researchers now believe that vitamin D from the sun is critical to health and actually prevents cancer overall, I was also shocked to find that there are (admittedly rare) cases of rickets in people who were on vitamin D supplements! The problem is some people don't absorb or metabolize vitamin D from food -- which isn't very surprizing, really, given that almost no food has any vitamin D in it (pretty much just some types of fish and a wee tiny bit in egg yolks). It's unnatural to get vitamin D from food -- the natural source is the sun.
Because of this I use very little sunscreen for my kids. Pretty much just a dab on the nose and ears when they're out all day. (There's also evidence that many sunscreens stop the red inflammation more than they actually prevent DNA damage, by the way, but that's another story.)
I try to make sure they get outside time in the sun regularly but moderately. They've never burned -- I'm still careful with shirts and hats to not overdo sun exposure. Burning's no good.
One of the replies mentioned getting UV through a window. It varies a bit, but most window glass blocks nearly all UV light, so I wouldn't depend on that. (They do it on purpose, otherwise your carpet's color would fade in just a few weeks where it's in the sun.)
The dermatologists, by the way, are dragging their feet on this. I've heard they don't want to reverse their official position because they're worried about being sued for telling people to stay out of the sun and causing deaths. Plus, those who aren't in complete denial mode are probably just embarrassed, I'm sure. People have a hard time admitting to mistakes -- especially people who are supposed to be experts.
I think this issue is kind of like the way doctors used to say margarine was better for you, although now it's clear that the trans fats in most margarines and shortenings are way worse than any other fat. And lots of natural fatty foods, like fish oils and nuts, are actually among the healthiest for your heart. Most doctors were about 20 years behind the evidence on that issue -- hopefully it won't take so long in the case of UV and vitamin D.
Here is a link to a popular article on the benefits of sun exposure:
Medical News: Vitamin D Benefits from Sun Exposure Outshine Cancer Risk - in Hematology/Oncology, Skin Cancer from MedPage Today
and here is a link to an abstract of a scientific paper on this subject in the Proceedings of the National Academy of Science, one of the leading scientific journals:
Addressing the health benefits and risks, involving vitamin D or skin cancer, of increased sun exposure ? PNAS