Explain Antibiotic Resistance to a Child

Start where the child already is: ask what they know about germs and about medicine that makes them better. Ten-year-olds usually know that some medicines "kill germs", which is the entire foundation you need, and starting there means the explanation is a small step rather than a lecture.

Give one metaphor and stay inside it. Antibiotics are like a key that opens the door of a particular germ; germs are not all identical, and the ones that happen to have a slightly different lock survive when the medicine is used. Those survivors are the ones that go on to make more germs, so over time more of the germs have the changed lock and the key stops working. Introduce "resistance" as the name for that, after the idea, not before it.

Check understanding with an open question rather than "does that make sense?", which almost always gets a yes. Asking what they think might happen if we use the medicine when we do not need it makes them do the reasoning, and shows you immediately where the explanation did not land.

Finish with what they can actually do — finish the whole course even when you feel better, never share someone else's antibiotics, and understand that a cold usually will not need them. Avoid jargon throughout: no bacteria, no mutation, no evolution unless they use the word first.