Good Son/ tell her to get a shorter skirt

"you think that would look good?" The girl asks, "well... OK I guess... wait here."

She goes back outside of the changing room and after about one or two minutes she comes back carrying a really short black skirt.

She slips out of her skirt and into her new one. You got to see her only with her panties on for a few seconds, but it doesn't really matter since the new skirt only barely covers her ass. If she was walking around in it you probably could see her checks.

"So... how is it?" she asks. You still aren't satisfied.

Good Son/ tell her to take her top and panties off

Good Son/ tell her she should try to wear more revealing underwear