I think donating them to the Goodwill or Salvation Army. There are girls who do shop for their dresses at these kinds of stores.
I bought a black velvet long dress with a white satin off the shoulder wide collar. I bought it for $10 so I could cut it up and use the cancan under it for a daddy daughter dance. My girl needed something to make her dress fuller and the bridal store didn't even carry the under skirts in black and they only rented them, for a starting fee of $25...ummmm, no thank you, I can make one by buying fabric at Hobby Lobby for about half that.
But then I found this dress and the underskirt was perfect. I cut it off the turned the top down, hemmed it, ran elastic through the casing, and we had the perfect underskirt. The dress is still wearable too, just not as poufy.