My first experience with Michael Morpurgo was reading War Horse with some of my extension reading group. The Butterfly Lion is definitely more accessible, but no less beautifully written.
Rating: 3.5 out of 5 stars. I easily read it in one session, but it didn't stick in my mind for long after.
Age suggestion: 8+
Ability level: From Grade 3 upwards for competent readers. More advanced readers will be able to pick up on some of the more old fashioned references and symbolism. It's only just over 100 pages and the writing is a decent size, and there are several pages that are just pictures.
Content notes: Morpurgo writes a lot about war. There's nothing overly graphic, but worth considering depending on the sensitivity of the child reading it. Conversely, it's a good text to introduce the topic of war without getting too intense.
If you haven't read anything by Michael Morpurgo yet, I'd highly recommend his work