A family calendar can be completely accurate and still fail, because the events are recorded and the logistics are not. Soccer at four and swim practice at four fifteen are both true; the fact that they are eleven miles apart and one adult is available is the actual problem.
An event needs an owner
The most useful single field on a household event is not the time. It is who is responsible for it. Without that, every event is implicitly assigned to whoever notices first, which is the same person every time.
Adding an owner also makes the conflict visible in advance. Two events, same slot, same responsible adult, is a clash the moment it is written down — not a discovery at four o'clock.
Say the handoff out loud
"Back by six" and "taking the car" are not calendar events, but they are the details that actually get missed. Somewhere to put a one-line note on an event, visible to everyone, absorbs most of the group-chat traffic a household generates on a weekday afternoon.