I have to second Bert.
Kids are known to "pocket" things, and after almost 6 years as well, I have never had one disappear. We are both in Pediatrics.
Another concern would be dropping off the desk and other trauma. I have gone through mice from "natural causes." I found an inexpensive site and bought a number of optical (LED) mice inexpensively as replacements, but have never lost one to theft.
