Have just found out that the CD of lullaby versions of Pixies songs which I found on Amazon is available on Spotify. It turns out to be only one of the massive Rockabye Baby series of lullaby covers albums, which includes all the artists you'd expect (Beatles, Stones, Bowie), and plenty more that you wouldn't (Bjork, Jay-Z, Tool).
I'm slowly working my way through them, putting together a Spotify playlist of my favourites (see below). It's interesting to find which tracks work best in lullaby form, as you're just stripping them down to their most basic melodies. I wouldn't necessarily have expected 'Wave of Mutilation' to be the Pixies track which worked best as a lullaby, whereas I knew 'We Are Going To Be Friends' by The White Stripes would be, as it's virtually a lullaby already. My favourite so far turns out to be 'Lost in the Supermarket' by The Clash.
I should point out that all these lullabies are instrumentals, otherwise the Pixies ones would be totally inappropriate, what with their lyrics about incest, and slicing up eyeballs and whatnot. I know Frida wouldn't actually understand, but it would still feel wrong somehow. Anyhow this whole playlist is of a lot more interest to me than her, these lullabies are yet to have any success sending her to sleep, but they are a useful distraction to me at times.