The Nottingham Post reported a single-cell battery was found inside a hollow milk chocolate Easter bunny in Bestwood, Nottingham, on Saturday.
Co-op has initiated a UK-wide recall following the alleged tampering incident.
About 3,000 of the items have been sold nationwide at Â£1 each.
The National Crime Agency and police are assisting with the investigation, the supermarket said.
Tammy-Louise Dundon told the Nottingham Post: "My daughter bit the ears off and shouted 'what's this?' I was just like 'oh my God'.
"How much damage do these batteries do? I've heard kids have died from eating them. They can get stuck in the oesophagus or can be corrosive."
A spokesman for the Co-op said: "The health and safety of our customers is uppermost in our minds.
"We are concerned about one incident of alleged product tampering involving our hollow milk chocolate Easter bunny foil figure, which has been found to contain a small battery inside.
"This follows an incident at Christmas when two similar products were targeted and contaminated."
Customers have been urged to return Easter bunny figures to stores for a full refund.
A criminal investigation has begun after a battery was found inside a chocolate Easter bunny at a Co-op store.