At the end of the day, ask your child to write the headline for it — six words or fewer, like a newspaper front page.
The constraint is the whole exercise: what was the most important thing, and what gets left out?
Write your own for the same day. Comparing two headlines about one day shows how much selection is involved in any account.
Summarising is choosing what matters and discarding the rest, which is far harder than recounting everything. A tight word limit forces that choice explicitly, and comparing two headlines makes it visible that accounts are constructed, not simply recorded.