Murdoch said her son also received a leaflet written for women who were pregnant. Murdoch said she would ask Canterbury A&P Association to investigate the stall and would lodge a complaint with the Advertising Standards Authority.
Avril Thresh said she saw young boys running around with the foetuses.
"Regardless of whether you support their beliefs or not, we don't feel it's an appropriate platform for them to be spreading their message."
Joanne Kirwan, another mother, said: "It's there for family entertainment, not for that type of message to be given to children."
Voice for Life spokesman Joseph Houghton said stall volunteers had behaved appropriately, but their use of the plastic foetus would be reviewed.
The Royal Agricultural Society is the umbrella group for 97 A&P associations. Society president John Grigg said individual associations each defined their own stallholder policies.
Last month, the Advertising Standards Authority upheld a complaint against Voice for Life about an ad published in Bay of Plenty media. The ad advised women to contact a pastor or priest, from a list provided, before undergoing abortion. A complainant raised multiple concerns and said the ad grossly misrepresented medical facts about abortion's risks and effects.