There are currently mainly CAD software packages on the market and recently many products have been launched aimed at children. Most of these are available free of charge and are designed to be used by children from 6 years of age upwards.
They typically have a fairly intuitive interface and the goal is to get children to use basic shapes to get them designing basic objects quickly to maintain their interest level. When first opening any CAD package it can seem quite daunting to a beginner (child and adult alike) as the initial view is shown in 3 dimensions i.e. the x-, y-, and z-axis. Although we see in 3 dimensions we are typically used to seeing things on a computer screen in 2 dimensions. Getting used to the 3-dimensional view takes some time but all CAD packages allow you to change between different views. This is critical to ensure that the individual parts are aligned.
One interesting free CAD package that takes a different approach than most is called LegCAD. LegCAD is based upon using lego bricks to build objects. The package is very interesting if your child (or you) is a Lego fan. The initial view is in 3D but this can be easily changed to a 2D view and it is very easy to rotate the view. The package includes a lot of the Lego bricks including special bricks found in the Lego sets for building cars, planes etc. These are found on the right of the screen. When selecting a lego brick from the menu a picture of the selected brick automatically appears in the top right-hand screen. It is then simply a case of dragging and dropping the brick onto the work space and moving it to attach to the other bricks. To attach to the other bricks you need to adjust the view to make sure that the bricks really are attached otherwise you are left with gaps between the bricks that may not be visible from the current view. The disadvantage of this is that you have to click on each brick to see what it looks like. Some are clear from their description but others are not and there are literally hundreds of bricks to choose from. If possible it would have been easier to display a small picture of each brick within each section on the screen (similar to Minecraft). This would make selection of the brick quicker and easier.
LegCAD gives the option to save the file in .STL format suitable for 3D printing but supports may be required to ensure that the model prints out correctly.
With little CAD experience it takes approximately 30-45 minutes to learn how to build your first lego model using the online LegCAD tutorial as a guide and the interface is easy to use.
Having tried out the package I think that it would take some time for children to build a model using LegCAD only. An easier option can be to get the child to build the object first using real lego bricks and then replicate the object in LegCAD.