My wife and son took a taxi from the Taxi stand near Waterloo street Guanyin temple on 11 May 2014 (Sunday) at around 3.20pm. They alighted at Mandalay Road around 3.35pm.
My son left his black zipper bag (with the name "Arthur Anderson" on the front of the bag) on the rear seat of the taxi. They couldn't quite remember which taxi company, but mentioned that it is Maroon in colour (which is then likely a taxi from SMRT).
They also heard the taxi telling the next passenger (that wanted to board the taxi) that he is going to Ang Mo Kio next.
Items in the bag include a library story book, and several pieces of Piano Music Scoresheets.
If you are the taxi driver that found my son's bag, please get in touch with me at rapt213@gmail.com.