Happy Birthday, Mrs. Piggle-Wiggle (2007) was completed by her daughter Anne MacDonald Canham based on "notes for other stories among her mother's possessions".

In the fourth book, Hello, Mrs. Piggle-Wiggle (1957) she has apparently returned to her original neighborhood and to her magical cures.

Some of the 151 children have the same names as members of the original audience (for example, there is a pair of brothers called Darsie and Bard).

This explains some of the inconsistencies, such as Mrs. Piggle-Wiggle being either a good witch/fairy or just someone who understands child psychology better than most professors of the subject.

The final book in the series, Happy Birthday, Mrs. Piggle Wiggle (2007), was published sixty years after the original, and is largely written by MacDonald's daughter, Anne MacDonald Canham (the two share a writing credit for this book).