Durable is about time. Sturdy is about build.
Because both words mean hard to break, children swap them freely. Ask one question to separate them: are we talking about how long it lasts, or how solidly it is made?
Durable means it keeps going through lots of use. Good school shoes are durable if they survive a whole year of playground football without falling apart. You only find out something is durable over time.
Sturdy means strong and solid right now. A sturdy stepladder does not wobble when you climb it to reach the top shelf. You can often see that something is sturdy just by looking at it: thick legs, a wide base, solid wood. Sturdy can describe people too, like a sturdy little toddler who keeps his balance on the bouncy castle.
Strong enough to last a long time without breaking.
Similar: strong, sturdy, tough
Strong and solidly built, not easily broken.
Similar: strong, solid, tough
Closest-meaning questions sometimes offer both 'long-lasting' and 'solid' as options. A child who knows that durable looks forward in time and sturdy describes how a thing is built can pick the right one instead of guessing.
Definitions and example sentences on this page are the ones a child sees inside Wordrealm, taken from our list of over 1,100 words. It is free to browse, with nothing to sign up for.
Plain-English guides to each test, from the councils’ own pages: Kent Test · Medway Test · Bucks Secondary Transfer Test