Brussels, Paris, or Amsterdam. Which is the best place for a family vacation?

While I'll agree that Brussels is not the most interesting city of the three you mention, I must just say that it's not without its attractions - if your family like beer (and as the youngest is 20, you're all legal in Belgium), then it's certainly the place to visit - the Belgians have a well-deserved reputation as among the best brewers in the world, there's a huge selection of wonderful beers to try out e.g. Chimay, Dubbel, Tripel, for something a bit different try a Kriek....