Staff at the kindergarten in the southern Swedish city of Kristianstad called the police after discovering the suspicious item in the evening, after children left for the day.
Once on the scene, police realized it was a grenade and decided to call in the bomb squad.
Detonation experts assessed the device — which a police spokesman described to CNN as “like a rifle round but bigger” — and deemed it too dangerous to move. It was then dismantled in a controlled environment at the kindergarten, the spokesman said.
The spokesman told CNN that the child had found the ammunition in a field used by the military for training exercises.
He said: “The military uses these big open fields for exercises, but at certain times of year they are open to the public when there’s no shooting going on and people are told not to touch anything out of the ordinary.”
Police did not identify the child, saying only that they were aged under seven — the starting age for school in Sweden.
The matter is now closed and not subject to a criminal investigation, the police spokesman said.
Things could have turned out very differently, Thomas Söderberg, the officer in charge at the scene, told CNN affiliate Expressen.
He said: “These are extremely dangerous things. If it had exploded when the children were around it, it could have gone really badly,”
“The child in question probably thought it looked cool and wanted to show it off. As I understand it, the parents didn’t know that they had the grenade at home,” Söderberg said.
“What happened is not good and I assume the parents will have a little conversation with the child.”
The condition of the grenade indicated that it may have been hidden for some time, the police told Expressen.
However bizarre the case may seem, it isn’t the first of its kind, according to Söderberg, who said that in recent weeks an elementary school pupil also brought a grenade into class after finding it in a field.
“He had put it up on a ping-pong table. A member of staff came in and almost had a heart attack,” said Söderberg.
The police were called and the device was found to be inactive.
CNN’s Henrik Pettersson contributed to this story.