To create a believable composite in Photoshop it's important to make a careful selection to start with. Here is an example of what can be done using a stock photo of a child looking "through" an ice cream cone and combining it with a new background.The basic steps would apply to whatever photos you decide to work with.

 (figure 1)

First I made a rough selection of the boy (figure 1) using the Lasso tool. Then using Quick Mask I tightened up the selection. When I was satisfied I copied it.

(figure 2)

I pasted the boy into my chosen background (figure2) and scaled it until to look realistic on the bench.When the boy was pasted it formed a new layer. I copied this new layer then selected it. I filled it with a color I picked up from the bench shadow then skewed it and scaled it until it looked like the boy's shadow. I used Multiply to get a transparent effect over the background. I adjusted the opacity and the color balance to match the other shadows. After selecting all I added some Guassian Blur to add to the realism. Moving the shadow layer under the boy layer in the Layers pallette completed the effect.

Click here for final composed image.