How do you tell grandkids grandma is a lesbian?

When they are ready. It is more important for the kids to have a good relationship with a loving grandparent than to discuss the grandparents sexual orientation. By virtue of the fact that they are a grandparent, they must have had heterosexual activity at one point. If the grand kids ask very specific questions, then answer them honestly without adding any opinions. Otherwise, wait until they ask. Just my opinions.