I've now read "Sellic Spell."
Tolkien thought it would be interesting to write a plausible reconstruction of a "folk tale" preceding Beowulf. Tolkien's story tells of the finding of a little boy in a bears'-den who grows to be a man of enormous power. Hearing of the haunting of the Golden Hall, he sets out and falls in with two companions, who unsuccessfully precede him in combat with the monster Grinder. Bee-wolf overcomes both monster and monster's mother and win's the king's daughter.
The story reads well; it is complete and there's no fumbling with details. I don't know what, if any, publication intentions JRRT had in mind for it. Christopher Tolkien dates it to the 1940s -- so it is something Tolkien worked on as a "break" from The Lord of the Rings, I guess.
I wouldn't say that only Tolkien could have written "Sellic Spell," but certainly it reflects his scholarly interest in the Old English masterpiece and his creativity too.
Yes: I could imagine an illustrated version of this story, published under a different name, as being a success with young readers, although it is just a bit gruesome.