Pick an everyday object your child uses — a lunchbox, a backpack, a toothbrush — and ask what annoys them about it.
Ask them to design a better one on paper and label the changes. Insist on a reason for each: why is that better?
Then play devil's advocate once: 'what happens when it rains?' Let them revise rather than defend.
Design starts from a real irritation, which is why this produces better thinking than an open invitation to invent. Being asked to justify each change turns decoration into reasoning, and one well-aimed objection introduces iteration.