How it’s done
On the weekly shop, part of the list passes into their hands with their own real budget: "the week's snacks get bought with this."
The rules: the money is genuinely finite (no rescue shows up at the register), comparisons are legal and welcome (price per unit, brand versus generic), and whatever's left over is decided together: do you save it, upgrade the shop, spend it on the treat?
Your role is available consultant, not auditor: you answer what they ask, you don't weigh in on what they don't.
What it builds — the why
Arithmetic with consequences — the only kind you remember — and the muscle of deciding between real options with limited resources. Also an early honesty about money: how much daily life costs is a fact many people discover too late.
How it changes with age
6–9 Childhood
10–12 Preteens
13–15 Early adolescence
What to watch for in your child
Watch what they do with the leftover: do they spend it right away, save it, invest it in upgrading the shop? There's their factory-set financial temperament — don't judge it, work with it. And be careful not to pass on your own money anxieties in your comments: the exercise is theirs.