She is getting the tape worms from the food she is hunting. She is getting them either directly from the mice or by ingesting flea eggs on the mice. You will need to treat mama and the kittens and then keep them inside to prevent them from continually getting reinfected.
Welcome to the poo watchers/worm fighters team ...
In fact Im sorry you' re having this trouble also :-(
I have a hunter here as well, she never picked a mouse , and I never saw a single flea on her but the other animals she hunts like lizzards or maybe birds ( never saw she doing but irs quite possible) are always bringing me problems.
Good luck, I hope you can find a way to avoid her eating those ( since mine like small lizzards, keeping her inside wont be of great use(if possible)