How can I remove these checkboxes for the child items?
You can override the CSS for the leaf node icon. For the default theme, the CSS class is called "scheduler_default_tree_image_no_children".
See also: https://doc.daypilot.org/scheduler/css-classes/
You can also generate a custom theme using the theme designer and remove the class from the generated CSS file: https://themes.daypilot.org/