I found, that in the file course/index.php we have the function "print_category_edit()", which prints the categories recursively, when you do Site Administration->Courses->Add/Edit courses.

Shouldn't we change the Line:

if ($categories = get_categories($category->id)) { // Print all the children recursively

to

if ($categories = get_child_categories($category->id)) { // Print all the children recursively

?