I'm 42, I have CF. I work in transportation construction. We move a lot of soil. Prior to going over to the transportation division I was a GeoTech. In addition to construction material inspection and testing I specialized in soil identification, soil stabilization, soil mechanics and foundations. It's a dirty job but somebody has to do it. (ZINGER! See what I did there). God made dirt, dirt don't hurt. The thing to avoid are molds. Late fall early spring, you can still happily play in material with decomposing leaves grass etc etc by wearing a simple mask, or even a bandana (and who doesn't like looking like a train robber from the Wild West). Sand boxes pose no threat unless a cat is constantly crapping in it as your playing in it. I have a garden and do my own planting, if I use compose I wear a dust mask as should anyone CF or not. Manufactured top soil I do not worry about. So long story short in my non-medical opinion, let her play, let her make mud pies, let her throw dirt clods, and come home in your favorite outfit covered in mud it's part of being a kid. And like any rational person when she is done have her wash her hands change her clothes put on an eye patch and pretend to be a pirate looking for buried treasure.
Best of Luck
Ed