To buy life insurance on someone you need an "Insurable Interest" in the person. This means you may suffer financially if they die, or that you rely on the person for financial support.
There does exist insurable interest between spouses, siblings, family members, and business partners, among others.
In addition, you may need to have your grandparents give their permission, sign the application for life insurance, and they may need to answer some health questions.
However, some forms of life insurance may or may not require the insured to be notified.
Learn more about how to buy life insurance for a parent or grandparent.