When they returned, they would try to hurt the living. Angry at their own short, wasted lives. These ghosts were called "utburd". This is a Norse word meaning "a child carried outside".
The child would attack his family first, and then any living person until they were destroyed. They could turn into wisps of smoke and go into any home. There they would turn into their true form - a withered frozen child. They would then crush or beat a person to death.
The only way to stop them was iron or a proper burial.