Naive is trusting because you have not learned otherwise yet. Gullible is being easy to fool.
Both words describe someone who believes what they are told, which is why children treat them as the same. The difference is the reason behind it, and how kind the word is.
Naive is about inexperience. A new Year 3 who believes the older children when they say the school pond has a crocodile in it is naive: she is young and has not met that kind of teasing before. Nobody really blames her, and she will grow out of it.
Gullible is about being easy to fool, whatever your age. If your uncle believes every silly story he reads online, even after being caught out a dozen times, he is gullible. It is a sharper word, and it usually means the person should have known better.
Too ready to believe whatever you are told.
Similar: trusting, innocent, naive
Believing things too easily because you have not yet learned how tricky people can be.
Similar: trusting, innocent, gullible
In closest-meaning and odd-one-out questions, both words can appear with trusting and innocent. Knowing that naive links to inexperience and gullible links to being tricked helps a child pick the option that matches the sentence rather than the one that simply sounds similar.
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