Found these completely by accident - the original samples used at the start of Acrylic Afternoons and throughout the entirety of Forever in My Dreams. The album they come from has just been reissued, though Jarvis or Steve (I imagine) must have had one of the original 1958 LPs arranged by Carl Orff - who of course was included on The Trip many years later.
Anyway the record is described very nicely here http://www.trunkrecords.com/turntable/music_for_children.shtml and I've added the tracks to the 2007 Yahoo account for you to judge for yourselves. I'm fairly sure I could never mistake the creepy children's singing from the start of Acrylic, and it's nice to finally know what they're saying!