Good physics makes good pictures of the world. At their best these are responsive, either evolving once started by their creator — predicting what does and what does not change over time, or laying down
the law, showing the possibilities and impossibilities in any situation as it is explored by the creator.
Comparing these created pictures with the real world decides their value. A good match and you have done a good piece of physics. That is, the imaginary world mimics the lived-in world. But it is only a functional mimicry: modelling has to be selective, as with any map-making.
This creative process can be hugely satisfying and computational modelling activities can be designed to allow children this pleasure. If so, children should be inventing and writing the rules to define what the model does, including making valuable mistakes. As with any open-ended activity, the skilful teacher guides progress through the activity, so that lessons can be learnt from false starts, and successes of many kinds appropriately celebrated.
Computational modelling tools can be used by the teacher to guide more convergent activities, constructing a pre-practised model with the class, or even to source or prepare a particular model before the lesson starts. Even at this end of the spectrum of use, there are choices to be made. With the completed model you can explore just the inputs and outputs, so using the modelling tool to produce simulations, or explore the structure of the thinking represented in the model with the class, so having a powerful and public way of sharing your thinking.
As its essence, a modelling tool provides a set of tools for expressing your own thinking, so making that thinking clear, especially to yourself. The kit-set of pieces and the ways in which they interact are made explicit: ambiguity is reduced. You need to be explicit about both your reasoning and about the representations with which you choose to reason: two hugely significant facets of modelling.
Computed worlds supplement word processing and drawing diagrams as a more dynamic way of expressing yourself with the computer: a modelling tool provides carefully controlled access to a wide range of expressive powers. So now you, and the children you teach, can create and share your own narratives.