Took me about two minutes of staring but I'm pretty sure I know how this was done. Very clever use of photoshop to create this while staying in the rules.
What can I say? This is not the best execution of a forced perspective shot, for one thing. Generally, you choose a very simple background so that you don't need to do all that Photoshop manipulation. What I'm seeing is a very heavy, rather clumsy use of the smudge tool on the background. I can't figure out why the bottom kid's head is flattened. I know it is supposed to look like the other kid is sitting on his head but it just looks wierd.The way the bottom kid's shirt and pants fit it looks like his head is twisted on backwards. Very disorienting. I'm guessing that you intended this to show Deep DOF by its necessity for this trick.