I have to be doing something wrong.
I am on the father's page who is connected to the children's mother.
I ADD a daughter, it asks me to choose the mother. It takes me to the ADD a PERSON screen. I enter the number of the child I want to add and get error "The specified number is already in use. Use F2 to find unused numbers."
(The end result is the same whether or not I choose a mother. Not sure whether I should choose one or not.)