You can blame my reading this story on G. K. Chesterton and George Orwell, both of whom wrote favorably about it. And a semantic note: toddler was a 20th century invention; at the time this book was written, "baby" extended up to the age of about five. Which helps explain the title with the youngsters involved being 3 and 5.
The narrator's sister and brother in law go off on a visit, and the sister, Helen, invites her brother to stay at their place. Give him a break from the city and let him read, and help scare off burglars just in case. Her two sons are little angels and won't be a problem. . . . (It is interesting to note that the parents leave before the brother arrives. The servants, after all, can look after the children.)
They are, needless to say. Endlessly, humorously, embarrassingly. Although their high-jinks do have one great side-effect.