It turns out, "Twinkle Twinkle Little Star" is the magic song that helps Simone stop crying. I don't know how Mary figured it out, but it works almost all the time. For example, last night Simone was having a little trouble staying asleep (really tired, but waking up periodically). She woke up really angry and I went in to check on her. As soon as I got to the line "how I wonder..." she'd stopped crying.
I'm sure the song will lose its magic soon and I'll have to resort to actual parenting, but it's pretty cute how she responds.