Thanks both, now is fixed.
Ok, I get it, if you select "add_child" inside a "profile", actually you are creating a new union with that profile as a parent and the new profile as a child.
When you add a child into a union, you just create the profile and not a new union. It is attached to the current union.
Is kind of confusing having a method inside profile called just "add_child" and not "add_child_and_union" or something like that.
This explains what I mentioned about lack of documentation for geni's API, I am missing a description of what the method is doing and what is good for.
Btw, I have noticed that the sandbox is providing wrong urls of profiles but the production server is ok.
Essentially it provides as url:
https:// api.sandbox.geni.com /profile-399
when shall be
https:// sandbox.geni.com/api /profile-399