This is the current eligibility requirements listed on the website.
Based on this, the parents must have or have had auto or property insurance with USAA. But I recommend calling USAA to clarify.
Thanks for the question. You can add your grandson in "My Profile"
Click "Add family Member", you can select grandchid in the drop-down.
I am double checking with an eligibility specialist to inquire whether you can open an account for your grandchild.