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

I think best way to create family tree starting from roots. First you enter oldest person you remember in a tree. And then you add children indicating only single parent for every child.

You do not need few types. You refer to the same type. the same type "Person" nas Child and parent fields and refer to the same type. This way every Person will show you who is parent and who are children all within single type.

And all you need now as a small custom one page extension that will use Cobalt DB to draw tree like view. That will not cost much.

Understood. In this particular case, the client wants to be able to input the children first, and then enter the parents next. In the actual database, we will only be concerned with children and parents/guardians. Once the user completes the child's name (and other fields), the user should be redirected to enter at least one family member (typically a parent). So while entering the parent, the child's name (or unique ID number anyway) needs to be pre-filled on the form and unchangeable.

I hope that made sense.

the client wants to be able to input the children first, and then enter the parents next

In most of the cases, when by the time you add Person there are not yet any children saved.

But anyway, why you use SQL source for that not relate fields?

Hmmm. Just to be certain, are you referring to the Relation Field Type? Or do you mean that I should simply create a List autocomplete field labeled "Related to"?

yes, that is exactly what I do refer to.