R-T, Margaret, and the Rats of NIHM
by Jane Leslie Conly (1990)
No wonder I couldn't find any sequels to Mrs. Frisby... on the "O" shelf: the next two books were written by Robert O'Brien's daughter. Not sure why after a Newbery winning book and successful (albeit altered) Disney movie he didn't write more himself?
While not as charming as the original, this book uses many familiar characters and situations, while focusing mainly on the human aspect: grouchy, lazy, overweight Margaret and her troubled little brother are lost in the woods, found and helped by the super-smart talking rats. With multiple references to pop culture it's a bit dated, and after the kids leave the rodents (why did the rats let them stay all summer?) and have to deal with family and reporters it's not as interesting, but still a good story for lessons on dealing with adversity, peer pressure, helping others, ethical dilemmas...