"What Happened on 9/11" is an invaluable educational documentary directed by Amy Schatz and produced by HBO. Unveiled in 2019, the film serves as a stark and powerful reminder of the tragic events of September 11, 2001, as it brilliantly grapples with the task of explaining these complicated and distressing historical events to a young audience. The documentary emanates from the perspective that for children today, especially those born after 2001, the terror attacks on that grim morning are history much like Pearl Harbor or Vietnam War - something they didn't live through and yet, it shapes the world they live in.

The film demonstrates great sensitivity in approaching such significant historical events. It begins by unraveling the regularities of an ordinary day, wove with old news clips, evoking the date's initial, innocent start before it became forever etched in the annals of American history. Through its 30-minute runtime, the documentary provides a balanced mix of historical account, personal narratives, and explaining the effects these events had on a personal and national level.

Importantly, "What Happened on 9/11" offers first-hand accounts from survivors and family members of the victims, further deepening our understanding of the events. These voices give an incredibly potent and palpable sense of grief that engulfs the participants, thereby transforming abstract history into personal stories. Their accounts are fleshed out through photographs and home movies, creating a poignant tapestry of memories that underscore the enormity of the loss.

In order to effectively elucidate the magnitude of the terror attacks and the reasons behind them, the documentary employs child-friendly language and explanations. It works hard to explain terms such as terrorism, Flight 93, World Trade Center, Pentagon, Al-Qaeda, and Osama Bin Laden, making the narrative accessible to younger viewers. It also successfully unravels the feelings of shock, confusion, fear, and profound grief that swept over the country in the aftermath, highlighting the unity and resilience of the American spirit in such trying circumstances.

The film endeavors to depict not just the terror attacks but also their aftermath, shedding light on the American experience post-9/11. This includes the effect on the Muslim-American community, a sensitive issue that "What Happened on 9/11" handles with remarkable poignancy, debunking stereotypes and bias in a simplified yet gratifying way for its young audience. It underscores the acts of discrimination that came about in the aftermath but also the acts of camaraderie and unity that defied those divides.

Moreover, while maintaining factual integrity, the documentary still manages to instill hope, focusing on how the country healed and came together after the immense tragedy. It showcases how the site of the Twin Towers was transformed into the commemorative 9/11 Memorial and Museum – a symbol of remembrance, pain, resilience, and rebirth.

The structure of "What Happened on 9/11" is designed to be child-friendly. Alongside the actual narrative, the film introduces children in a classroom setting, raising questions and discussing their knowledge of the events. This classroom aspect works effectively in breaking up the intense historical recounting and creates a sense of identification for its young viewers.

Overall, "What Happened on 9/11" is an essential teaching tool that navigates the complex narrative of September 11 in a respectful, subtle, and strikingly honest way. It exposes younger audiences to a monumental event in American history, not through the lens of horror, but through an empathetic, thought-provoking, and informative approach. A riveting blend of archival footage, personal narratives, and straightforward explanations, this documentary stands as a formative viewing experience for younger generations, urging them to remember, understand, and learn from the past.