It really depends what you mean by 'traditionally'. My reading of [1] is that the 'ka' reading is one of many although it's become more prevelant in modern usag

It really depends what you mean by 'traditionally'. My reading of [1] is that the 'ka' reading is one of many although it's become more prevelant in modern usage. [2] says that 'papa' was most common in Old Japanese. [1] https://en.wiktionary.org/wiki/%E6%AF%8D#Japanese https://en.wiktionary.org/wiki/%E6%AF%8D#Japanese [2] https://en.wikipedia.org/wiki/Mama_and_papa https://en.wikipedia.org/wiki/Mama_and_papa