If you know nothing relevant, you're ambivalent between exchanging and not exchanging, which is what "fungible" means.

I see where you're coming from, but I'm not sure that's a useful use of the concept of "fungible". The reason why it's useful to think of dollar bills as fungible is because we can use them to think about more complicated exchanges (i.e. dollars -> cow -> cow + milk -> cow + dollars -> ...) and figure out a net result by comparing before and after quantities of the highly fungible little notes. Sure, it's possible that a given dollar might turn out to be counterfeit, but it only takes a little knowledge and a few moments of examination to become very confident about whether or not that's the case for a given note.

On the other hand, I'd be very reluctant to use babies as a unit of exchange even if I ignored the obvious moral problems: babies are much much harder to compare for equality than dollars.