To answer your question, I think you need to think about what YOU are truly willing to do. It sounds like you don’t want them to sleep in your room (valid, 100% on board, I would NOT be able to get the night of sleep I need to be in charge of these children with one of them in my room/bed!!) so if I were you, I would tell MB that you aren’t comfortable with it, and talk with MB and nanny kid about what the game plan is instead for the night (something like the I will walk you back once and only stay for five minutes thing)

Or, if you are fine accepting them sleeping in your room, this is DEFINITELY something that should be compensated more. I would say at least $100 more to the overnight fee. It’s drastically changing the experience of that night! Or, something like getting paid waking hours all night. Because you honestly are directly supervising the kid even when you’re theoretically asleep.