How do you spell 'papa' like what some people call their grandpas?
You just spelled it correctly - PAPA.
Origin: French; Middle French pappa (nursery word); compare Latin pāpa father, Old Norse pāpi, pabbi father.
Another variant of the word "papa" is spelled as "poppa". Merriam-Webster and other online dictionaries have an entry for this word, referring to the informal way of calling your father.
What Would Grandpa Do? Read ten things that our grandparents might have done more than us that we can all do now in order to live a little more consciously at One Green Planet.