This thesis presents the work of developing and testing a programming tool specially designed for children. The purpose of this tool is to make children capable of creating visual simulations and games that consist of actors interacting in a virtual world. The name of the programming tool is Behavior Cards, which comes from the fact that actors are programmed by giving them cards representing different kinds of behaviors. Behaviors are to some extent pre-programmed and can include general events specifying how the actors should act in different situations, in contrast to specifying every possible situation that can occur. The idea is that Behavior Cards should be easier to use than a more traditional text based programming tool, but it should also be expressive within the domain of visual games.