Sun Burn Stickers for Kids

We hit the pool one last time on Monday in celebration of the close of summer.  As I lathered up the kids with SPF I had a stroke of genius.  What if there was a color change waterproof sticker you could put on your kid before you put sun block on.  Then if at any time during the day the block wore off the sticker would change to a bright red, letting you know you needed to put more sun block on that kid.