10 month old unlocking iPhone
There's something like a 1:50k chance of someone having a fingerprint that can unlock your phone, according to Apple. 

Those odds drop dramatically for a relative - though right now I cannot find a citation for that, I did read it on teh interwebz so it must be true ;-)
Googling finds a million scare stories but I can find no hard fact to support my claim.

I'd say you just have to keep watching her until she does it, see if her fingerprint can actually unlock your phone. [I'm not experienced enough at parenting to suggest you make her do it repeatedly to see if it works.]  

The chances are very much against her guessing the passcode - it would be far more likely that she'd activate the timed lockout instead.