There are at least a couple of options for private tours. It implies that you may be the only ones, potentially, but at least you'd have a leader to show you around, and if you have others they could come too.

Firstly, there's the [Rotterdam Walking Tour][1]:

> Discover the secrets of Rotterdam with a knowledgeable local guide
> (3hrs walk)

And more promisingly, [Gilde Rotterdam Guided Tours][2]:

> Gilde Rotterdam organizes city walks guided by senior citizens. These
> guides will elaborate from personal experience about life in the city
> and about the differences between Rotterdam of old and the modern-day
> metropolis. There are several walking routes in all parts of the city
> and two bicycle routes.


  [1]: http://www.toursbylocals.com/RotterdamWalkingTour
  [2]: http://bit.ly/12KG1Ue