It's pretty simple and would be a great project to do with the kids. I paint the clothes pins in the primary color twice so that it really gets saturated.
Next I make the larger spots using the eraser tip of a pencil. They come out slightly different depending on the angle of the pencil and how much paint is on the eraser. To make the smaller circle I sharpen the pencil down a bit and then break the lead tip off. Again, it depends on the amount of paint and the angle that determines the dots.
I hope you enjoy these as much as I do.