Furtive is about hiding what you do. Sly is about being clever at tricking others.
Both words describe someone sneaky, which is why children use them interchangeably. One is about secrecy, the other about cunning.
Furtive behaviour is secret and usually a bit nervous. A furtive glance at the biscuit tin, then another over the shoulder to check nobody is watching, tells you someone is hoping not to be seen. A furtive person does not need to be clever at all, just quiet.
Sly is about trickery. A sly player might offer to pick the teams fairly, then quietly make sure all the best runners end up on his side while looking completely innocent. He is not hiding: he is fooling everyone in plain sight.
Done secretly, hoping nobody will notice.
Similar: sneaky, sly, secret
Clever at tricking people in a sneaky way.
Similar: sneaky, crafty, tricky
Character questions often turn on words like these. Furtive suggests guilt or nerves, while sly suggests cleverness and a plan. Choosing the wrong one changes what you say the character is like.
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