You can assist them in preparing their tax returns, but it is imperative that they consent to you doing so, and that they have the mental capacity to understand that they are consenting.
If they do not consent, or do not have the capacity to consent, then it would be fraudulent (illegal) for you to prepare and submit their returns, unless you have Power of Attorney for handling their financial affairs.
Make sure you provide your name on Form 1040 as the person who prepared the return for them. And keep copies for you and for your grandparents.
