"She got on the carousel, bent down and picked up a baggie full of a white substance. My guess is it's a bag of dope," the woman told KTVI-TV, St. Louis.
The woman, whose identity was not released, accused the restaurant manager of failing to handle the situation properly.
"I brought it to the manager's attention and he didn't want to call the cops or anything," the woman said.
Chuck E. Cheese released a statement saying that law enforcement has not verified the nature of the substance that was found.
"We take great measures to provide a safe experience for the children and families," the statement said.