6 and 7 year olds.
This is a lesson I learnt from Joehttp://artwithmrhall.blogspot.co.nz/2012/06/primary-colour-gradients.html
and I have used it before with an older group of children.
These little guys did a stirling job.
The children painted an A3 page with one primary colour, across into a different primary colour. This passes through a secondary colour which they created along the way. We talked about not being able to see the change in colours. Once dry, they cut the page in half, lengthwise. They drew circles on the back of one half. They used lids and containers to draw around. These were cut out and placed on the first half. They were asked to put them on the contrasting part. They found this to be intuitive.
They drew their origami aeroplane which they made earlier, with pen. They cut it out and added it to their paint blending picture.