When a person is stored and also a partner is associated to this person and the child is already stored, it is possible add a child to this parents. To do that klick on on the right inside the frame of the children of this partnership. Now the search view opens and above the search you will see the data of the parents in the information frame.
Here you can search for the child. A click on a result entry joins the child to that parents. After that the detail view of the children appears.
To remove or edit an existing ancestry, click on below the frame of the mother at the right side. Here you can edit the ancestery or delete it with the button "delete".