To delete a group / class, three conditions must be met:
the group must not contain any students;
there should be no teacher assigned to the group;
access Kinderpedia from a laptop / computer using a web browser (Chrome, Mozilla, Safari, etc.).
After these conditions are met, go to the "Children Groups" section on the left side of the page, and next to the group you want to delete you have a red button that looks like a trash can. To delete the group, click on that button.