Well, you could always use like Papa Clark and Nana Lois for them. And then Grandpa X and Grammy Y for the next generation.
Another route would be to use traditional "grandparents" names from other cultures and languages. Like Omi as friends of ours call their grandmother. (I think this might be German, with Ompa being for Grandpa.) One generation could be in English, another in German, and another in French or Spanish or Greek or Russian, or Thai, etc.
