Lars and Shelly Brosdahl called police, who confirmed that the substance was methamphetamine, worth up to $200 on the street.
Someone who looked like a teenager dropped something into their son's bag as he went trick-or-treating with his 9-year-old sister on Halloween night, the Brosdahls say.
"He said some bigger kid ran by him and asked if he wanted some candy," Lars Brosdahl said. "He said `Sure,' and the kid dropped it into his bag."
The clear crystals looked like rock candy, the parents said.
"The (kids) could have OD'd on it. That's what makes me so shaky and upset," Shelly Brosdahl said.
Police think the young man was a suspect fleeing police after a report of an assault in the area that night.
Police in Ramsey, northwest of Minneapolis, did not immediately return a call seeking comment Tuesday.