Yaya is also used in Spain, though more so in Catalonia. It makes much more sense phonetically for toddlers as they first begin to enunciate, by following the repeated 'consonant-vowel' formula, as in Mama, Papa.

Or Nana in English, but as 'n' and 'm' are so phonetically similar, it distinguishes more effectively.

Long time before my wife and I will have to choose or grandparental noms de guerre (noms de coeur?).