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:
Discover the secrets of Rotterdam with a knowledgeable local guide (3hrs walk)
And more promisingly, Gilde Rotterdam Guided Tours:
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.