A four-year-old child took about 250 packets of heroin to her nursery and began passing them out, believing them to be chocolate, Delaware police say.
Police say the child unknowingly brought the drugs to the centre when her mother gave her a different bag to take to nursery.
The girl's mother, Ashley Tull, 30, has been charged with child endangerment and a drugs charge.
Her sister told the Associated Press she had no idea the drugs were inside.
Alicia Tull said her sister does not use or deal drugs and that she was taken advantage of by an individual who stored the drugs in her home without her knowledge.
Delaware Police Master Cpl Gary Fournier told the Delaware News Journal newspaper the "investigation is continuing and more charges are forthcoming" but would not say whose heroin it was.
Several children who received the packets went to hospital as a precaution, police said, but none of the packets was opened.
In total, the bags held nearly 4g (0.14 ounce). Each packet was labelled "Slam".
Ms Tull has been released on a $6,000 (£3,700) bond has been ordered not to have any contact with her children, ages 11 to four, who are in the custody of a relative, Delaware State Police said.