Apartment in Köln

Finnish family is searching for a long term rental apartment in Köln. Preferably not too far from University, e.g. Sülz, Klettenberg, Lindenthal, Zollstock.
We are moving to Köln because I start a new job as a researcher in the University of Cologne. My working contract starts on 15th of March 2018, but it is also possible to move in the beginning of March or in April.
We are family of four people, our two kids are 1 and 5 year old. We are very clean and accurate tenants, non smokers, have no pets. Our desired apartment is at least of two rooms, with kitchen. Minimal size is 60 m2.