So after the Peer Review it was discussed that the extra additions that the kids add to the kids would appear to be drawn on.
I found a tutorial on Video Co-pilot which talked me through how to unveil lines using a kind of mask which follows the original image line. This way I could create the idea that the image was being created as the kids were thinking it; mirroring the concept of the features being drawn on as the kids compete with each other.
Of course only using the 2D character images we have, the test looks a bit flat. A test with a 3D rendered image would be more useful. I also need to find some PS brushes that have a more crayon-like appearance.