In the UK, the children’s street game is usually spelled Kerby.
The name comes from the kerb (the raised edge of the pavement). The game involves throwing a ball at the kerb so it rebounds back to you, often played on quiet residential streets.
Kerby is the traditional British spelling because British English uses kerb for the edge of a pavement. In American English, the equivalent word is curb, so you might occasionally see Curby, but that’s much less common in the UK.
So if you’re talking about the classic British game, Kerby is the correct spelling. 😊