The lunch aide, who was placed on administrative leave, gave the students the treats Thursday during recess.
One student said they thought they were being given normal cookies.
"I was upset that the one that we are supposed to trust in school and us supposed to care for us would do that to us," the student, whose name wasn't reported, said.
On Friday, school officials sent an email to parents saying the treats came from a pet store.
"I don't know what possessed her to do something like this," said Lynn McGuckin, a mom who served as a lunch aide at the school for four years. "We weren't allowed to hand anything out to the kids."
School officials said they notified authorities about the incident. The aide's name wasn't reported.