Selecting a category in checkbox is selecting all categories

I have a custom listing form where you can select multiple categories in the “Field Category” with the setting at Checkbox. I have five main categories with 10-20 sub-categories in each parent category. If you check any sub-category, all is good, but click a parent category and all sub-categories are checked.

If I use ajax, multi-select, radio, they all work correctly. But my client wants Checkbox.

Also, the parent categories are not displayed or highlighted as a parent category, they are just a straight long list. Can this be formatted?