The point is that Duo only has a certain number of words programmed for each translation. I guess granny just doesn't happen to be one. You're right about it being dated-although my grandchildren still call me "granny'. Anyway stick to the more usual vocab to get through smoothly. See here for some tips:
Ogni is not plural. It means "each", and it comes from the Latin omnis. Just as in English you wouldn't use it with plural nouns such as "each apples" or "each men" or "each days", in Italian you don't say "ogni mele" or "ogni uomini" or "ogni giorni". Like "each", ogni is used with a singular noun, not a plural.