Yes it is possible she was mis-diagnosed - yes, it is possible to only have the visible outbreak after 15 years, though she may have been harboring the virus.  I am interested as to what your doctor is saying regarding the 2 vaginal births (I assume they were vaginal) and the possible exposure of both children to herpes.

There are situations where it seems 'impossible" but a spouse exposed for years still comes up negative.  I would practice safe sex with a condom from this point on and be attuned to when your wife has a recurrence with a visible rash so that you can refrain from sex during that time period.  I would also talk to the doctor about anti-viral therapy.