well, you don't know that it's not exactly necessary to literally eat from a toilet seat in order to get infected with, for example, salmonellae? There's a reason why many people don't really sit on public toilets but bend over backwards instead and then pee.
By limiting the access to a toilet in a work place you basically limit the risk of getting ill, yes, believe it or not. Otherwise we could all offer our bathrooms to strangers who come a long and can't find a public bathroom anywhere close by.
I understand that many of you here are shocked about the bad shop assitant not letting the cute little one use their bathroom. Maybe try to understand the worker's postition as well.