I put a link to kojo in a comment on Phil Wadler's blog where he has been discussing languages for kids

Great!

Needless to say kojo doesn't have a low ceiling.

Exactly right. Kojo's got a low floor and a high ceiling (courtesy Scala).

From your comment:

My 7-year old grandson loves it, but it does need a bit of adult expertise to explain error messages at the moment. The author is working on it.

I'm afraid this is on hold right now due to time/prioritization constraints.
After I added Stories to Kojo, the subset of Scala used by kids became much bigger (and the task of defining language levels for Kojo became much more complicated).
Also, the kids that I work with seem to get by just fine when they encounter errors. Most of them don't pay much attention to the error messages, but, given that they know (with Kojo's help) the exact line number where the error is, they are able to examine the location of the error - and (mostly) figure out and recover from the problem.