Tap Check in at a handoff. The time is recorded in the family record and your co-parent can see it.
The value is in doing it every time, not only on the days something goes wrong. A record of forty on-time handoffs and two late ones tells a story. Two entries, both made in a bad week, tell a different and less convincing one.
Location
You can attach your location to a check-in. It's optional every single time, it's only ever captured at the moment you tap, and the app never tracks you in the background or between check-ins.