Urban Dictionary is unhelpful in relation to the etymology of momo. This is the best I could find:
http://forum.wordreference.com/showthread.php?t=1800803

I'm not convinced. However, the CBeebies programme Show Me Show Me has a character in it called Momo. So I think you're okay for now*
http://www.bbc.co.uk/cbeebies/show-me-show-me/songs/show-me-show-me-momobot

*CBeebies might be having an Enid Blyton moment