Because of strict privacy rules in the Netherlands, living people are not easy to find. In general, records of people born less than 100 years ago are not public.
Here are some sources for researching people in the 1900s that may help you to find living relatives.
Another option is to take a DNA test. Because Dutch people are careful of their privacy and most already have pretty complete trees, DNA testing is not that popular. See the Dutch DNA testing strategy for tips on maximizing your chances of finding Dutch cousins through DNA.