To Make a Clean Sketch

1. Open a colour image.
   
2. Make a copy of it.
   
3. Make the Copy a greyscale image by de-saturating it. Ctrl Shift U.
   
4. Make a copy of the Greyscale image, Ctrl J.
   
5. Invert the copy of the desaturated image by Hitting Ctrl I.
   
6. On the Layers Pallet change the normal drop down from Control to Colour Dodge. (The whole canvas goes blank at this point.)
   
7. Now, go to filter, Blur and select Gaussian Blur, add a small amount to suit as you look at the now re-emerging image. ( 2 to 4 pixels should be enough)
   
8. Now flatten and save the image.
   There you have it a grey scaled line image of a picture, ideal for the kids to colour in.