Passports! See question bellow.

With both of you being Colombian citizens it's probably not necessary to get passports -
even though it may prove to be a good idea to have them just in case.  I would certainly get them if I were in your situation. 

Rather than rely upon advice given in this forum, if I were you I'd contact Migración directly and get all your questions answered.  If everything is on the up-and-up you should have no fears about getting involved in "something funny", since I understand that Migración is not known for their sense of humor...

http://www.migracioncolombia.gov.co/index.php/es/

The top menu bar under Atención al ciudadano lists services and has a Preguntas frecuentes document available as a PDF.  At the bottom of the page they provide an email address as well as telephone numbers for inquiries.

I agree with OsageArcher, I don't think a Passport is needed quote wikipedia '...Citizens of Colombia do not need a passport when traveling to Argentina, Bolivia, Brazil, Chile, Ecuador, Paraguay, Peru, Uruguay and Venezuela. For these countries, they may use just their domestic identification cards called Cédula de Ciudadanía.".
However I would double check, and whether you contact Migracion by email or in person, ask for the reply in writing, just to cover yourself.