Deceiving is tricking someone into believing something untrue. Betraying is breaking the trust of someone who relied on you.
Children often use betray as a stronger word for lying. The two do overlap, but each can happen without the other.
Deceive is about a false belief. A fox in a fable who pretends to be friendly so that the hen will come closer is deceiving her. You can deceive anyone, even a complete stranger, as long as you make them believe something untrue.
Betray is about broken trust. If you tell your best friend a secret and she announces it to the whole class, she has betrayed you, even though she never told a single lie. Only someone who was trusted in the first place can betray.
To trick someone into believing something that is not true.
Similar: trick, fool, mislead
To let someone down by breaking their trust, often by giving away their secrets.
Similar: double-cross, deceive
Comprehension questions often ask how one character has treated another. Betray tells the reader there was a relationship of trust that has now been broken, which is a much bigger point than a simple trick, and answers that notice this tend to be more precise.
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