I'm going to tell the babysitter's mom that she put a bottle in the microwave
Wait, did you open the bottle in the microwave?
I did it without knowing the plastic would melt from high heat. She's so reckless!
It's reasonable to expect the babysitter to properly take care of your children. Telling their parent may get them in trouble, but could be for the best if they're doing something that is potentially dangerous.
I'm going to tell her mom then.
Well hopefully this sort of dangerous mistake doesn't happen again. I'm glad that you're looking out for the children that your babysitter watches.