How do I insert hidden form fields or disabled text fields in forms?

Let's say we wanted to build a family tree, starting with the children. This is what I would like to do:

Allow the user to input the child's name in the first form.

After that, the user can enter his/her parents or other siblings.

So after entering the child's name, the next form should contain a field that has the child's name pre-filled in a text box on the next form. (Or the child's name can be stored in a hidden field.) This way, we know that the next person inserted is related to the child.

How would I create a form that is pre-filled with a default (but unchangeable) value? Would the value be passed through the URL or some other way?

Thanks.