I had two broken arms as a child (though NOT at the same time!) - at the age of 5, I broke my wrist clean in half and had a cast up to my shoulder for 6 months! The situation was VERY ITCHY (LOL) - stick an unsharpened pencil, a drinking straw, anything with a blunt tip down or up into the cast, and she can scratch with that. However, it is important that whatever you guys choose to use as the "in cast scratching instrument" is BLUNT - if she scratches too hard, she can break the skin inside the cast and cause all sorts of skin infections and/or scarring.
Has she had her cast changed yet? Usually, with school aged children, the cast needs to be taken off and replaced at least once during the duration of wear as they can get VERY gross and stinky - if she has not yet had it changed and is at least halfway through the wear time of the cast, it might be time to contact the orthopedist and have it changed. She can get a good scratch in when they take the old one off and put the new one on. It may be itchy because its yucky in there!