Paramedics tested children at an inner-Brisbane childcare centre on Friday afternoon after a quantity of drugs was found in a playground area.
Five ambulances attended the Lady Gowrie Centre in Fortitude Valley - near Brisbane's nightclub precinct - and tested 36 children.
The drug is an anti-convulsant, a spokeswoman for the Queensland Ambulance Service (QAS) said.
None of the children showed any symptoms, she said.
Queensland Health decreed the children could go to their homes once assessed.
"The parents of these children have been provided the fact sheet and are advised to monitor their children," the QAS spokeswoman said.
"Should they have any concerns Queensland Health advise that they should contact their local doctor or the Poisons Information Centre on 131126 for further information."