As of the 6.x.1.2 version of Node Relativity you cannot have underscores in the "machine readable" content type names that you want to use with this module.

For example, if you have a content type called "my_child" that is the child of a content type called "parent", you will have to rename "my_child" to something without an underscore, such as "mychild".

If you leave the underscores in, when you try to create your child node from the parent node, you will be sent to the generic Create content page. This is useless, because even if you try to create your child from there the parent/child relationship will not be established. Furthermore, if you checked the "Enforce Parental Rules" box in /admin/settings/relativity/advanced, your child content type won't even be available!

In the meantime, just get rid of those underscores and everything will be fine.