Love the fact that I possibly got an STI from my own mother, despite never having had sex at the time ... Then again, I can never know if I did get it from my mother - it could have been either of my grans, my grandads, aunts, uncles, cousins, the stranger who asked to hold and kiss another stranger's baby, a little child who asked to hold a baby, a stranger who had previously kissed another stranger, the latter of which then touched their face before proceeding to shake the hand of someone else who then touched their face and then so happened to kiss me at some point before I turned 5 years old etc ...
The definition of an STI is ... wait for it ... "sexually transmitted infection". If you have not caught an infection by having sex, then for you, it is NOT a sexually transmitted infection - rather, just an infection that may just so happen to be transmitted sexually but not in all cases (same way as HIV can be acquired via a sexual encounter or a transfusion with tainted blood ...)
Just to put further heeby jeebies up you, you do realise that a LOT of people have the virus but NEVER ever break out in a single cold sore? Then again, others, like me, get them up their noses so they are not visible to anyone. Others get mouth ulcers which are also not visible to others (yes, just in case you were wondering, they too could be caused by the same virus) ...
I can completely understand you not wanting her to kiss your baby when she has an active outbreak. However, you then should not be allowing ANYONE to kiss your baby. Also, have you personally been tested for the cold sore virus? Has your wife? Do either of you never get mouth ulcers? If the answer to any/all of those questions is "no", do you kiss your child? If so, you are personally putting your child at risk ...
After what may seem like an unnecessarily pedantic post, I guess what I am trying to say is that minimise risks all you want - there are just some things that are not ascertainable or preventable and you would do better to apportion and act upon any worries you may have about your child proportionately ...