I've been there 3x (2000, and 2x in the early 90s). Key West is generally very safe for visitors. Basically, the only risk you run into is if you lose control of yourself such as excessive alcohol/drug intoxication and losing control of the situation as a result, or stupidity in other acitivities such as diving into water, driving jet skis at unsafe speeds, etc. As long as your daughter is sensible, she should be quite safe.