Opening with a no-win action sequence works for action movies. Consider the audience that likes testosterone-fueled adrenalin sequences and their expectation to get to what they paid for quickly. It must involve melodrama shorthand because it is a mini-story with characters who are instantiated and dispatched within a single scene: villains in black hats doing villain things, and parents being killed protecting children who watch from the closet with louvered doors or from under the bed. I'm not suggesting you would go with the worst cliches, but these opening scenes play out so often I don't believe they have any narrative impact whatsoever. They just show us how the villain kills to establish a pattern. If your project is not an action movie, I think I have a better suggestion.
The events may be a case of common knowledge among the characters and so they don't discuss it, or discuss around it because it is too sensitive. Knowing something profound happened that has left a wound might have more impact for the reader to see the wound first. The details of the incident can be trickled over time and from different viewpoints (including unreliable narrators), and provides opportunity for real character building when events in the current action start to open that wound and they must discuss events that have been emotionally buried.
If we have already seen the full narrative arc of those dead parents, created and killed in a single scene, those characters are done for us and there is no mystery. They were just the soup before the meal, and we have emotionally moved on even if the main character insists he can't.
I suggest instead to withhold the parents from the reader so we have no preconceived idea about them. They are as absent from our lives as they are to the characters, and we discover them through this wound. We are aware of them, but we don't get to know who they were. Instead we feel their loss through the other characters, and the lingering dysfunction reveals the hole where something use to be. We don't get to make up our own minds and close their story. In narrative sense they are still characters that influence the thoughts and actions of others, and "alive" because they can still grow through the opinions and maturity of the survivors.
Also consider if there is something unresolved about the event, potentially with the help of unreliable narrators, the reader can experience their own version of not being able to get over it. If re-examining the question results in different answers, even hostility, we have some empathy for the character who can't let it go. We also have a multi-faceted event seen through the lens of different characters each of whom has different emotional coping skills.