Early in 2020, the Baring Foundation published Creatively Minded: An initial participatory mapping of arts & mental health activity.
That report identified around 20 organisations or projects working with children and/or young people UK-wide. This is probably also an underestimate but their impression was still that there are relatively few organisations in this field.
This collection of case studies of arts and mental health projects with, by and for children and young people aims to showcase the work of some of the organisations identified in initial participatory mapping.

Lowe, H.(2020). Creatively Minded and Young. UK: Baring Foundation.