These bikes (which, confusingly, are also called bakfiets in Dutch) are specially-made with a wood cargo box designed to accommodate children, groceries, and all the other necessities of a parental commute or errand-run. The cargo box has a seat for kids, and some even have seat belts.
The Bakfiet brand cargo bikes, with handcrafted wood boxes, are handsome design objects in their own right, and cost between €2,000 and 3,000. I've also spotted cargo bikes by Babboe that look lovely as well.
If you're visiting Amsterdam, you can rent the bikes at various locations. In the US, Dutch Bike Company in Seattle imports.