I have never seen a childrens paint program that dealt with vectors at all. They are all about allowing the kids to throw paint at the canvas in a variety of fun ways, with an eraser and perhaps an undo button.
However, a completely separate paradigm I have seen is object/sprite based; in this case, there is no ability to arbitrarily paint, and instead the child can choose among a wide variety of props to place upon the scene.
The former mimics real drawing and painting; the latter mimics playing with tangible toys. Both aim to be easy for children to use by drawing heavily upon the real world activity they emulate. Both avoid involving the child with representational models or indirect manipulation. For example, to make a red car you would get the red paintbrush and click the car. You would not select the car and alter its properties to be red. This type of indirect action represents the computer's internal model of the data, but is a poor representation of the way a child approaches the task.
However, this is contingent upon age; as children mature, they are able to handle more complex internal models of an application; I would not have most 6 year olds deal with vectors and property dialogs, but I wouldn't be surprised if this is fine by the age of 10.