How big is your ball? I'm assuming its still quite small if you're feeding it mice. If you are worried about safety offer your ball get some rat fuzzies/pups. They are about the size of adult mice and will generally be less of a threat to your ball, not to mention rats are more nutritious than mice and making the switch to rats is a lot easier when they are young versus trying to switch an adult mouser.
On that note, it's also fairly easy to switch them to frozen/thawed prey at a young age.