If you wish to delete a class (lesson) off the only one chart (child), you have to go neither to Class Schedules on the right-sidebar nor to Class schedule page (see two screenshots below).
Deleting a chart off these pages causes the removal of a chart schedule (all classes which appear on the class schedule) itself. Please see the example:
By clicking either on Class Schedules on the right-sidebar or on Class schedule page, the following window will appear:
It shows that two charts (A.M. and A.I.) are assigned to one class schedule (A.M.’s Schedule).
The class schedule consists of two classes: Activity Works and 1st Core. If you delete a class say Activity Works on this page, it will be deleted on both charts (A.M. and A.I.).
In order to avoid this, please do the following:
Click on a chart, and then delete a class from there by clicking the “x”.