Yes, a parent may have Insurable Interest on his or her adult children, which is required in order to buy life insurance on someone else.
However, you may need your child's permission, and signature on the application for life insurance in order to buy life insurance on your child.
But, many parents do buy life insurance on their adult children, especially if they have paid for the adult child's college education using loans, or if their adult child has their own children but cannot afford to buy life insurance on him or herself to provide financial security for their own children.