While anyone can visit the Kinderdijk World Heritage Site for free, you must pay an entrance fee of six Euros (four Euros for children) to visit the Visitors Windmill, which is officially called Nederwaard Windmill 2. The Visitors Windmill is open daily except during the winter and early spring season, November through March, when it is only open on weekends.
It takes about five minutes to walk along the canal to the windmill from the parking lot. The walkway is paved and flat, but you will have to walk up a few steps to the windmill itself. Inside the windmill, the only way to see the different levels is to climb up and down steep steps that strongly resemble the "ladders" on military ships.
The entire Kinderdijk windmill area has been designated a UNESCO World Heritage Site.