Many parents do purchase life insurance on their adult children to provide life insurance protection and financial security for their grandchildren, in case their adult children can't afford life insurance.
In addition, many people buy life insurance on their adults kids to guarantee repayment of their college loans they made to their children.
In order to buy life insurance on someone you need an insurable interest in the person you wish to insure. Parents do have an Insurable Interest in their children, and may purchase life insurance coverage on them.
Learn more about if you can purchase life insurance for an older child.