D.,
Perhaps you could use the bags that come on home-delivery newspapers to keep them from moisture/rain. They are L-O-N-G PLASTIC BAGS that you could open at the other end, slip up over her cast, and then tape at the top and bottom. Use a couple of these bags together to ensure water resistance. "Daily" bags are probably large enough, depending on your daughters age and cast size, but I know that the "Sunday Edition" bags are even large enough to cover an adult's arm cast.
Hugs,
K.
P.S. - If you don't take home-delivery newspapers yourself, ask a neighbor for a few bags. They will be GLAD to give you some, because they pile up like crazy. I keep a bin full to use to put "wet" trash into my garbage -- coffee grounds, potato peels, celery trimmings, and bones, (basically anything I don't want to run through my garbage disposal).