children is a list of Child objects on Father so children will be the first child in the list and children will be the second and so on. So children.childId (assuming there is a childId field on the Child object) will be its id and children.childName will be its name.
The way you are doing things now your Father object is your form backing bean. I was under the assumption that you were just going to bind the form fields to all the attributes on the father and his children (some of them will be hidden) then when the father is submitted back up to the controller you will call merge and update what is in the database. Is that correct? If it is then i would loop over the collection of children and show each one on the page allowing them to edit whatever they want (remember to bind every field to the form even if its a hidden field or else when it is submitted back up to the controller it will be null).
something like the below snippet:
Of course here we have only the childs name so this would not make the page to busy, but if there were lots of other editable fields you might want to do it differently. Maybe in that case you would want a page that lists a fathers children and when they select one you submit that childs id up to the controller and return a view that takes the child as the form backing bean and you edit just that one child's attributes and then update it. Once you get the basics down you might want to start playing around with ajax and data tables too.