Cargo bikes are dangerous for children, there is little protection in case of an accident (not necessarily with a car, a bollard is enough) and well once you have one child in, you can't really have groceries. You can't use it when it's freezing. What you describe is typical of a one-child family in Netherland, but doesn't fit the reality of most of European cities. Also Netherland has a rather high amount of cars per head, so not everyone thinks like you ;-)