We visited just before Easter and my three sons loved it (they are 9, 5 and 4). The museum is a showcase of science and technology and great for children to understand the how, why and what.
During the weekend of the 23rd and 24th of April there's even more reason to check out Science Center NEMO It's the grand opening of the Energetica exhibition on the museum rooftop and entry to the museum is free. Expect it to be busy though (remember the importance of gratis in the Netherlands?)!
The museum is within walking distance of Amsterdam Central Station.
I'll let the photos do the rest of the talking!