Start from what they love

A child who loves swimming, horses or a video game already lives among numbers: laps and times, hands and furlongs, levels and hit points. A problem about that world starts with their nouns and their numbers, not yours.

So the first move is a question, not a problem. Ask what numbers come up when they do the thing they love, and write down what they say. You are collecting raw material; nothing is being tested yet.