Open Messages, write, and send. There's one thread per family — no folders, no subject lines, nothing to file.
Sending writes the message into your family record with a timestamp and a fingerprint. From that moment neither of you can change it or take it back.
What the other parent sees
The message appears in their thread, and they get a notification if they've allowed them. Notifications never show what the message says.
Writing a message is not the same as agreeing to something. If you need a decision recorded, say so in plain words: "To confirm: you'll collect Ava on Friday at 6."