R-T, Margaret, and the Rats of NIMH, written by Jane Leslie Conly, involves two children, Artie (or R-T) and Margaret, who got lost in Thorn Valley's woods and stumbled upon the Rats of NIMH. They make friends with a rat named Christopher, and live with them for a while, before returning to the world of men.
The dynamic of this book is that, once they return, Artie and Margaret must keep their discovery of the Rats a secret, so that the Rats don't get sent back to NIMH.
A somewhat interesting book, but it is written for children, and not many fans of the original, Mrs. Frisby and the Rats of NIMH, appreciate it much.