A good question.
A local answer.

A casual breakfast café near the day's first attraction can save a cross-city detour.

Protect enough Sydney nights to enjoy the city and one nearby outing, then treat the northern stay as a separate flight-and-hotel block. Count both travel days before deciding whether three or four nights there are worthwhile.

Choose a base by the teenagers' likely activities and the walk to a useful station or ferry. Check the apartment's real bed layout and laundry, then plan evenings around venues that are actually open on your dates.

Check current train and attraction hours before fixing the mountain itinerary.

Give Sydney several relaxed days for the harbour, neighbourhoods and a Blue Mountains outing, then choose only one additional region if travel time permits. Hotel changes and flights consume more of a short post-cruise stay than the map suggests.

Use luggage storage only after confirming size limits and collection hours for the departure terminal.

Check whether the pool is included for your room category and if children have separate hours.

Book early for a special night and mention any seating preference rather than assuming every table has the view.

A baby carrier can offer more flexibility if your group is comfortable using one.

Keep the car pickup location in mind, but do not let one departure morning outweigh five days of sightseeing convenience.

Compare change fees and accommodation costs with the value of one extra performance.

Read recent reviews for noise and lift reliability, especially in a busy waterfront area.