AITJ for expecting full payment when the baby falls asleep while babysitting?

I (19F) babysit occasionally for a couple in my neighborhood. They’re nice and the kid is sweet it’s usually an easy gig. Last weekend they asked if I could watch their 2 year old from 6-10 PM while they went to dinner with friends. I agreed on the usual rate and showed up at 6.

By 6:20, the baby was already passed out in my arms after a bottle and a quick cuddle. I laid her in her crib and didn’t hear a peep the rest of the night. Literally just sat on their couch scrolling my phone for almost four hours checked the baby monitor a few times and that was it.

They came home around 9:45 and were surprised the baby had been asleep almost the whole time. Then the husband half-jokingly said, “Dang! maybe we should just pay you for the 20 minutes of work” and the wife laughed. I laughed too at first thinking it was just a joke but then they handed me cash for one hour and said, “Honestly we feel weird paying full price when you didn’t really do anything”

I told them I still came over they still had their night out, and part of babysitting is just being there in case the baby wakes up. They didn’t budge and said they’d “make it up to me next time” but I left feeling kind of taken advantage of. I didn’t argue further because I didn’t want to cause a scene.

When I vented to my sister she said they were being cheap and I was totally right to expect full pay.

AITJ for expecting full pay even though the baby was asleep almost the whole time?