Then I would think an "I am going out to do X - can you watch the child?" would be sufficient.
I have usually been with planners, so we would set our schedules for the week on sundays. My person in particular liked calendars so we had them hanging all over the house and we would mark down our events so we both would know where the other one was and when. This included the grocery shopping that I usually did because that way we would know the night by which to make the list so if either of us wanted something specific we knew when to mark it down.
The advantage to this was that it made a lot of the sorts of exchanges people here report finding annoying completely unnecessary.