Watch Singaporean families on a Saturday and you'll notice they don't do heroic days. They do two-hour blocks, punctuated by air-conditioning. This is the two-hour rule, and it is the single most useful piece of local knowledge a visiting family can import.
The mechanics are simple. Heat and humidity tax children roughly twice as fast as adults, and the tax arrives suddenly — a cheerful kid at 11:50 is a carried kid at 12:10. So you cap outdoor blocks at about two hours, and you put something cold between them: a hawker lunch, a museum, the cooled domes, a mall, the hotel pool, or the oldest and best cooling system of all, a nap.
A worked day
Out by eight-thirty for the outdoor block — the zoo at opening, a neighbourhood walk, the promenade — while the light is still kind. Cool block from noon to three: lunch, then one indoor anchor, then rest. Back out at four-thirty as the heat breaks; the second window runs through golden hour into the evening, which is exactly when the bay, the wheel and the light shows come on. You will have done two solid things, seen the city at its two best hours, and nobody will have been carried.
Reading the warning signs
The tell isn't complaining — it's when a child goes quiet and stops asking questions, usually an hour before any adult notices the heat themselves. That's the end of the block, whatever the itinerary says. Water constantly, hats on the unshaded stretches, and remember: the city is built for this. You are never more than ten minutes from somewhere very cold indoors.