What is the Greek word for little grandmother?

There isn't a single, universally accepted word in Greek for "little grandmother." Greek, like many languages, would express this idea descriptively rather than with one specific diminutive. You could use several options depending on the nuance you want to convey:

* γιαγιάκα (yayiáka): This is a diminutive of γιαγιά (yayiá), grandmother. It's the most straightforward and commonly understood option.

* μικρή γιαγιά (mikrí yayiá): This literally means "small grandmother" or "little grandmother." It's grammatically correct and easily understood.

* You could also use terms of endearment along with γιαγιά (yayiá) depending on the relationship and the speaker's regional dialect.

So, while there's no perfect equivalent, γιαγιάκα (yayiáka) is probably the closest and most natural-sounding option.