I don't know if this would work, but when DD wouldn't wear hats, we turned it into a big game. For a couple of weeks we put everything on our heads, called it ajar and fell about laughing. Now she loves hats, or even things that aren't hats, but could be put on your head and called a hat. When we go shopping she actually points out all the hats and wants to try them on!

The other thing they could try is getting some glasses for a doll, and talking about how cool/clever/ smart etc the doll is in her glasses. Again, not sure if it'd work, but these re the first ideas that sprung to mind.