A witness to the controversial slapping of a four-year-old boy at an Ellenbrook shopping centre says she has been left "devastated" after police advised her that charges against the mother would be dropped.

WA confirmed on Thursday the common assault charges would be dropped.

The woman, who wanted to be known only as "Janet", gave a harrowing account of the alleged incident to 6PR Drive host Adam Shand on Thursday, saying she was astounded to witness a distressed young child be slapped twice across the face at the busy shopping centre.

Janet said she noticed the woman pushing a shopping trolley with three children, one of whom was walking behind the family, crying and distressed.