How do kids address or refer to their great-grandparents, if they are still alive? (equivalent to "dad", "mom", "grandpa", "grandma") "Great-grandpa" sounds rather lame.
Here in Pakistan, there are specific words that kids use to call the great grand parents.
However, as of what I know, there's no specific word in English which can be used for great grandparents.
It varies according to the culture and to the family. In some families, you would notice kids calling both their great grandparents and grandparents by one single name, that is "Grandpa" and "Grandma". In some, I have seen kids using specific words for their great grandparents like "Grandma Norma", any cute and simple word is used..
It is a rare situation. It is unusual for a child to have a close relationship with a great-grandparent. The answer will vary from family to family.
I knew one of my great-grandparents. I called him "Pop"* It was his nickname in the family. In other families a great grandmother might be called "Nana" or "Ma" and so on.
For a learner, this is even less of a problem. You refer to your great grandparent in their language, not English. And when talking about your family you refer to them as "great grandfather" or "great granddad" or "great grandpa" (depending on your relationships)
* If I called him anything at all, which was rare, for an 8-year-old, a 90-year-old man is unimaginably ancient, wrinkly and really rather scary. He was certainly too old to "play" with, and 8-year-olds don't really enjoy sitting beside a bed and chatting.