I found this comment in the code of the WooCommerce Composite Products-plugin:
Inherit coupon validity from parent:
– Coupon is invalid for child item if parent is excluded.
– Coupon is valid for child item if valid for parent, unless child item is excluded.
Thus, a workaround is excluding the category(ies) of the child products from the coupon.
In case of a fixed cart discount this would invalidate the coupon. To solve this you must tick the checkbox “Allow discount on cart with excluded items” on the usage restriction-tab.