A good question.
A local answer.

Choose the destination by its station-to-attraction distance, not just the rail journey. A beautiful region can still require a long local transfer.

Choose the rides and one walking area that matter most rather than buying a pass and trying to do every track.

Choose one main walking section and a clear turnaround time before adding extra lookouts. Rail travel uses a substantial part of the day.

The ferry ride and island can make a varied half day if your child enjoys boats and open spaces. Choose a few stops rather than trying to cover everything.

Stay behind barriers and check weather and path notices. The coastal edge is exposed even on a pleasant harbour day.

Confirm with the airline whether bags are checked through and whether you must clear immigration, collect them and change terminals.

Build in time for a short walk or meal near the wharf, and check the last convenient return before leaving.

Use the current journey planner for the route from Circular Quay to your exact hotel; airport-area hotels can be near different stations or shuttle stops.

Price a direct airport transfer against rail for all six people and bags. On embarkation day, allow time for check-in and terminal queues.

Eastwood and Cabramatta offer different food communities, but check travel time from where you stay and any specific dish you want.

Choose one sit-down meal to enjoy and keep other meals simple with cafés, bakeries or a market. Check the full menu price before entering.

Check how you will get home after dinner. A compact walk around one neighbourhood is more enjoyable than crossing the city for several venues.