When we were searching for a preschool for our daughter, we started with simple search engine queries using the word "preschool" with our city and state to find local preschools' locations and contact info. Then, we called each of the schools to learn more about their enrollment policies and program offerings. Next, once we had narrowed our search to a handful of schools that really interested us, we researched these preschools in parent listservs and online forums (such as Berkeley Parents Network and Bay Area Parents), to see what other parents had to say about the schools (community reviews). We also visited the potential preschools to see the: learning environment; safety and cleanliness of the facility; and how well children in them were responding to the program and adult teachers/role models (we were reviewing a diversity of preschools, including parent-run co-ops, as well as Montessori and teacher-run schools).
I'm including some resources below that may also be helpful to you and your family as you research and secure a preschool for your grandchildren.