The best way to treat a stain is to find out what kind of stain it is (e.g. grease, protein, fat, dye).
I have a few that will absolutely NOT come out of our kids' clothes despite being relatively simple (chocolate).
For grease, the best suggestion I've heard and used is dish soap on the stain or something like Goo Gone.
For protein, hydrogen peroxide is often most effective (though it can have bleaching properties)
The Queen of Clean is a great site, but I HATE that she charges for advice. So, if you're willing to cough-up $3 for her advice, you will likely be able to save your clothes.
Don't shy away from contacting the manufacturer of the sunscreen via their 800 number either and ask why the sunscreen is staining the clothing. They may have had similar issues and may have a simple, free solution.