Racket would be my suggestion. As much as I like Common Lisp, it has a lack of material targeting kids (other than particularly motivated ones). Scheme would be a second suggestion, but Racket is descended from Scheme and so is mostly (with a bit of effort in some cases) compatible with any Scheme learning material you might want to use. Scheme also requires you to decide which implementation to use, which can impact available libraries and tools.
It's also very easy to get visual results in Racket which often helps to keep kids motivated (seeing results of their work quickly and easily, versus needing to jump through hoops or read lots of documentation just to render a single image).
https://racket-lang.org/
It's also very easy to get visual results in Racket which often helps to keep kids motivated (seeing results of their work quickly and easily, versus needing to jump through hoops or read lots of documentation just to render a single image).