I have PMLE (Polymorphic light eruption which means my skin turns into a blistery bubbly itchy rash after just a few minutes of sun exposure!) and I ride a bicycle for days at a stretch in the sun so I have to something that will not only not let me burn, but won't even let me tan. In addition it has to last for hours, I have to be able to sweat through it, and it can't be greasy. Tall order? Yup - but I found the stuff...
http://www.solsunguard.com/kindersport.html
Not the cheapest, but worth every penny! I have a local bike shop which sells it and will be happy to mail order it to you if you cannot find it anywhere else - PM me if you need the info.