Yes, that occurred to me too. It seems like the "five beer bottles" was a surprise - was the LW upset that her parents drank her beer? Did they not drink when she was a child, and so she thought that was because children were around, not because of some other reason? (It was too expensive, one or both of them was on medication that precluded it...) Or was she just upset because she would (quite reasonably) expect a paid babysitter not to drink, and didn't realize she needed to spell that out for her parents? (Not to mention that paid babysitters are not infrequently below the legal drinking age, in the US! Even a college student might be under 21, and babysitters in my experience are very often high schoolers.)

(Also, "finishing off a six-pack" in Prudie's answer felt really weird. What if the LW's parents bought the beer and took the other one home with them, or perhaps, thoughtfully, left it in LW's fridge? Five of six isn't finishing off a thing, it's either taking almost all of a thing, or not finishing it!)

(Also also, if I were babysitting an infant, I'd be more likely to have one beer than if I were babysitting a toddler, but not because the toddler's more entertaining to me - the toddler's also more likely to try to drink my beer.)