Hmmm. I don’t remember any specific piece for learning the pedals. most easy pieces do not have a whole lot of pedals in them. There are some exercises for using the pedals in the Yolanda Kondonnassis book “On playing the harp”.
Salzedo’s METHOD FOR THE HARP is the perfect book for getting used to the pedals, and has the ever-popular “Chanson dans la Nuit” and “La Désirade” in it.
The Bach Prelude in C is easy as far as the fingers go, has changes on all pedals, and has the benefit of being recognized and useful as a perfomance piece.