"They've got no sense of time"

You said five minutes and it was twenty. You said we're late and he carried on tying his shoe as if the word meant nothing. From an adult's chair, time is a thing everyone shares, and a child who ignores it is ignoring you.

Sylvie Droit-Volet, in a 2013 review, went through the research on how children perceive time. Infants have a primitive sense of it. Through childhood, the ability to judge durations changes considerably, and she draws a distinction between implicit judgements of time, which come earlier, and explicit ones, estimating how long something took or will take, which come later and keep developing. The review looks at which brain systems involved in timing mature faster and which slower to explain why children's time judgements differ from adults' in the ways they do.

I won't put ages on it, because the review doesn't give thresholds and I'd be inventing them. But the direction is plain. A nine-year-old's sense of what five minutes is, and of how far away twenty to nine is from half past eight, is not an adult's sense with a bit of carelessness added. It's a developing judgement, and "we're late" is a sentence that requires it. To act on "we're late" a child has to know what time it is, know what time they need to be somewhere, work out the difference, compare it to how long the remaining tasks take, and feel the urgency that the difference implies. Adults do all of that without noticing. Children do it slowly, badly, or not at all, and the last isn't defiance.

There's a second thing in "five minutes". To a child, five minutes said by a parent means "not now, later", and later is a place rather than a duration. When you say five minutes and come back in five, you're keeping a promise in a currency the child doesn't yet hold, and their surprise that the time has gone is genuine.

The arrangement for this is the simplest one in the book and it's in chapter 19. Make time visible. A clock they can see, a timer that counts down where they can watch it, and "when the timer goes" instead of "in five minutes". You're not lowering the standard. You're converting a thing they can't yet perceive into a thing they can see, and a child who can see the time run out is a child who can act on it.