In an interview Fitzgerald gave toward the end of his life he stated that every child who read his books was his child.
John married a wonderful woman name Joanne. They had a long and happy life together, but no children. John and Joanne were a fantastic uncle and aunt to John's nieces and nephews. Nearly every Great Brain book is dedicated to one or more of his nieces and nephews.
I was only able to locate one nephew during my research. We wrote a couple of times. As the years went by the family lost touch with other, even today I cannot locate the other cousins who might fill in more of the later story.
One of my readers asked if he could write a note of gratitude to any remaining children, since your children are his children, wrap them in a bug hug and give them a kiss for reading the stories he left as his legacy.