Back when I was sitting on other people's kids the thing I used to hate the most was when the parent who was leaving would spend 5 minutes explaining to the toddler that they were going to leave him alone with a stranger and working the kid to a frenzy instead of just saying it and leaving. It took forever for the poor kid to settle down when they did that and just a few minutes when they made it a casual fact. I always felt sorry for the half-crazed kid.
In the end most of them are pretty amazing, clever, and a lot fun one you've worked past the screaming thing.
Food seemed to be the way to get them to stop crying.
I can see that working well...
I assume you weren't handing out spinach and brussels sprouts?