Prefer weekdays and opening time
Prefer weekdays if you can, with fewer lines at the ticket office and food courts. In Santiago, spring weekends get crowded from noon, so arriving at opening time gives you two quiet hours. That window is worth gold with kids because they walk faster, see better, and reach popular sectors without pushing.
Check the official calendar for school events and holidays. Some parks receive massive school visits in spring; if you encounter one, reverse the route and start at the end opposite the large group. Arriving early also lets you choose shade for lunch before 1:00 PM.