I think it's more likely to be a fictionalized version of historical events. Sunday schools usually aim to teach moral and religious values, and while Hitler's actions are historical facts, the 'Hitler Sunday School Story' would probably be crafted to fit the educational and moral goals of the Sunday school. So it would take real events like the Holocaust and Hitler's rise to power and turn them into a story that emphasizes certain values like justice and compassion, rather than being a pure historical account.
It could be either. It might be based on real historical events related to Hitler but presented in a way suitable for a Sunday school, which could involve some simplification or moral emphasis, making it somewhat of a fictionalized account of history. Or it could be a completely fictional story that uses Hitler as a symbol to teach a moral lesson.
It depends on how it's presented. If it's told as a way to accurately convey historical facts about Hitler within the context of a Sunday school, then it could be considered a real historical account, albeit a simplified one for the intended audience. However, if it's more of a parable or a story that uses Hitler as a character to illustrate a moral or religious point, then it's more of a fictional story. For example, if the story focuses on how Hitler's actions go against the teachings of a particular faith, it's more about using the historical figure in a fictional narrative to make a point.