Thomas Söderberg, the officer in charge at the scene, told the Expressen newspaper that the event could have had a very different ending.
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."
The case follows a similar incident in Sweden weeks ago, when a primary school pupil brought a live grenade to class and placed it on a ping-pong table.
A member of staff came in and almost had a heart attack," said Söderberg, who noted that device in that case was found to be inactive.
"What happened is not good and I assume the parents will have a little conversation with the child."