In reply to:
To be clear, I've always assumed the refrain at the end is a jumping-rope rhyme. Historically these have the same sing-song quality and projection of fear, and start with "My mother said..." (e.g., "My mother said/I never should/play with gypsies/in the wood.") So it's not just a mother's warning; Major Tom has become a childhood bogeyman, even.
my same thoughts. I've heard two live versions of the song (yet to hear the album version), and during both it sounded like more of jump rope rhyme. Especially in the Serious Moonlight version, when Bowie's sitting on the ground kind of rocking back and forth.