We were in Amsterdam two years ago, when my youngest was 4. It was OK, but not fantastic. We went shopping, pottered about in street markets, had pancakes on Damm Square , and a boat trip. The boat trip was good as it was raining by then, and we were under cover! The really great thing about Amsterdam is that it is tiny, so everything within a 4-year old's walking range.
We did Zaanze Schaans the following morning but got caught in torrential rain - so bad that I actually stripped off to my bra in the carpark.
We have just had a 3-day trip to Paris and it was fine for young children. We had museum passes for adults, but they are all free for under-18s, so we could breeze into places without feeling that we had to do the whole thing.
We did a Seine boat trip, which my little ones loved, climbed the Arc de Triomphe, and visited Versailles (as well as going to le Louvre, Orsay, Notre Dame and Pompidou. We also strolled around Concorde/Tuilleries (sp), where there was a lovely fair with very impressive rides.