These windmills (molen) date from the 1740's and were used to pump water from the polders to dry out the areas below sea level.
You can visit the interior of one of these 18th century structures to see the workings and how the people lived in quite a cramped space. It was very windy the day we visited and the sails were in full flow which was a nice sight to see. Entrance is 3.50 per person and 2 euros for under 17 year olds. There is no charge for just walking along and looking at the windmills
It's an interesting walk but you can also hire bicycles for a few euros if you want to go further. It will probably not take you more than 90 minutes to walk around and see the inside of the visitor windmill. There is also a small canal boat ride every 30 minutes.
From Rotterdam by the Erasmus bridge there is a boat which will take you all the way to the Kinderdijk. Probably a good way to get there as parking at the K'dijk costs 5 euros and it takes a while to get there by car or other public transport.
If windmills are your thing then definately go.