how to add a child to father who is currently not married to child's mother
First attach the child to the couple. Then edit the relationship of the child. That edit button a pencil-and-paper icon next to the child's name under Parents (in the web app, at the bottom of the Details page; in the mobile app, under the Parents tab).
When you open the relationship you will see the parents expand above the child and under each parent is a link to edit just that relationship. That gives you a pull-down menu where you can select Step.
@Ronnie Ellen Kingsbury
Welcome to the "Community.FamilySearch" Forum.
I am just another 'lowly' User/Patron ...
Just in passing ...
Further to what has already been proffered ...
[ Please be, aware; and, advised, that a Couple "Event" (eg. Marriage) for the Parents, is necessarily NOT required... ]
Here are some "Knowledge Articles", in 'FamilySearch':
I hope, that this may help/assist, somewhat.
Once you are finished getting the relationships right, it looks very nice. I wish Ancestry had a display that was as easy to understand in a glance. Here is an example of how easy it is to get everything correct.
Thank you everyone for your help. I am new to this and it's nice to get answers.