The other day on Facebook John Thorn mentioned the future publication of a biography of Cool Papa Bell by Lonnie Wheeler. Won't be out until February of next year but the publisher's web page states it's the "first full biography." ETA: Just re-read your post about it needing to be something a seven-year old can appreciate. Hopefully there are lots of pictures. A terrific book aimed at youngsters is We are the Ship by Kadir Nelson. The artwork is superb.
https://www.abramsbooks.com/product/...9781419750489/