On the "Batman 1966" Facebook page, John Stagman asked:
In the episode "The Bat's Kow Tow," Commissioner tells Bruce that his grandchildren told him he needed to go see Chad & Jeremy. How is that possible when Barbara is his only child?
This is an easy one to answer. Barbara was his only child by his wife at that time. He must have been married before, or he had at least one other child outside of his marriage. This child, as of 1966, was grown up and had several children whom Gordon acknowledged as his grandchildren.
This splainin only works for Earth Prime-Time. It has nothing to do with the "Batman" legend in Evil Toobworld, as seen in 'Gotham' and it's definitely disconnected from the universe of Comic Books.