The general steps I followed to get to the final result were:
It all begins with intention. Nothing complicated in this case. Just wanted to convey a sense of cuteness. The other intent was to echo the theme of the birthday, which will be at Rainforest Cafe.
My wife came up with the brilliant idea of having a picture of him peeking from behind a tree. With that concept in mind, I started visualizing the shot. I wanted the tree to be on the left side, with him leaning out from behind the tree with a mischievous, teasing smile. He would be in the tree's shade but illuminated by a soft light source from the right, and the grass behind them would be in backlit sunlight. We had a park near our house that I thought would have the right kind of background.
I thought of how we could take the shot. With our son in the shade, it would hopefully be easy to control the quality of the light on our son. To get a soft light I thought about using the sb-800 behind a shoot through umbrella on a light stand from a higher angle so that the direction of light would look somewhat natural. I don't have a softbox with grid but if I did, I would've tried that instead. As for the ratio, I was thinking of using the umbrella as key and underexposing ambient by 1 or 2 stops to act as fill.
The day of the shot, we dressed up our son in red, which I thought would contrast nicely with the green grass. I got home just in time to get to the park a little more than an hour before sunset, i.e., close to the magic hour.
So that was the idea and planning stage of taking this shot. The end result isn't what I had originally envisioned but I think it turned out ok. I'll take about execution and postprocessing in my next entries.