Adult children may have an Insurable Interest in either or both parents.
In order to get life insurance on someone else you must have Insurable Interest in the person you wish to insure for life insurance.
In addition, you need the insured's permission and their signature on the application for life insurance coverage.
Since adult children may either rely on a parent for financial support, or be responsible for paying for the cost of their parent's final expenses (burial and funeral), there does exist an Insurable interest.
Learn more about buying life insurance on your father.