OK then - how do you do that? My son2 would love to know...

First you need to take a photo from far and above a scene.

Then, you post-process it.
To get the effect, you need a soft like photoshop (but any software with blur effect will do).

With photoshop, you push the Q letter to get the quick mask mode.
You select the gradient tool, and you pick the reflected type.

You do a gradient under the quick mask mode.
Then you type Q to quit the quick mask mode.

Now you have an image with 2 parts selected.
You go to filter, blur and select "lens blur".
You apply with the parameters you need (add blur level as you like).

You may use Gaussian blur under other tools.

It computes for a while. Et voila !

It gives a "toy" rendering to picture, and it is cool !

Classic tips! Many thanks - will let Son2 know... :-)