Depends on how old she is... they don't play as hard when they are younger.
If you are going to wrap it, I would use bubble wrap, if the small bubbles then wrap it around 2-4 times, if the bigger bubbles then once is probably enough... then tape it on, packing tape would be fine.
When my son was 4 and playing there was a girl on his team in a cast and they wrapped it with bubble wrap then put a sock (parents) over the whole thing so that it stayed on. I think they also covered her hand with something so the bubble wrap didn't bother her.
Why not let her play........ football players play with soft casts all the time, what is the difference if you are making the cast soft with the wrap?!