Demonstrators apply different sunscreen and skin lotions on the frisbee's white disc, and this is then exposed to the sun, to see if the lotions have really worked. Obviously if there is enough protection, the areas would remain white and not turn purple.
The EPA is targeting children because sunburn takes nearly 20 years to develop into skin cancer. The danger lurking, is that UV rays are capable of damaging skin within 15 minutes of exposure and just five or more sunburns could elevate a person's skin cancer risk by almost double. It would really help, if children were aware early even before they reach their teens about the dangers of sunburns so that they could adequately protect themselves.