How many kids did mrsfrisby have?

Mrs. Frisby, the main character in "Mrs. Frisby and the Rats of NIMH" by Robert C. O'Brien, has four children: Teresa, Martin, and two others named Timothy and a third unnamed sibling. Timothy is particularly important to the story as he is sick and requires special care. Mrs. Frisby's determination to protect and save her family drives the plot of the book.