I just looked up this problem last week. :) Believe it or not, a bar of Ivory soap.
"Warm water and rubbing Ivory bar soap will get out sunscreen. It works for fabric softner stains too. Wet garmet, rub on Ivory bar soap, work into lather on spot, agitate with hands a little and rinse. Repeat if nessessary, launder as usual."
It worked! Hope it works for you!