I wonder if the children were William's from a prior marriage. There is a marriage for a William John CRICHTON in 1925 to a Cathrine Ann PEEBLES.
Catherine died in Jan 1937, but there was some dispute over the will and the articles below mention that Catherine had two sons from a prior marriage and William had a son from a prior marriage also:
http://paperspast.natlib.govt.nz/cgi-bin/paperspast?a=d&...--
http://paperspast.natlib.govt.nz/cgi-bin/paperspast?a=d&...--
The property mentioned in the articles, 51 Eden Vale Road, is the same address that William John Crichton and Caroline Crichton are living at in the 1954 electoral roll.