As fill, just throw it on there in TTL BL mode and it should be fine.
If you want to use it to light up your subject a bit more, and underexpose the background a bit, set your camera to -1EV and the SB-900 on the hotshoe to +1.3EV and you're all set.
I've not found the diffuser (the supplied one, for all intents and purposes, is identical to the Stofen) to be useful for nothing other than eating about a stop of light outdoors.
That's it really, and it eats more than a stop!
If you think about how a Stofen etc works, if there is nothing for the light to bounce off outside, then it's not going to do anything because the size of the light source itself is no bigger at all. All the softening comes from the light bounced back from the surroundings, mainly a ceiling.
There have been a few posts here about Stofens doing at least something slighly useful outside, but that really isn't my experience at all. The theory is that there is usually something nearby for the the light to bounce off - other people, the ground, whatever - and that will provide some soft fill.
Well, to all intents and purposes, it doesn't. I've tried it every which way, using my trusty model - an upturned wheelbarrow with a pineapple on top, in the garden
The only way I could get the Stofen to show any noticeable difference in that situation was to put a white towel on the ground as a reflector and even then the difference was almost invisible.
However, it must be said that the Stofen never does any harm, and might just do some good if your subject is standing in the snow surrounded by druids - you never know. And if your gun is big enough to have a bit of power in hand that you can afford to waste, then no harm done at all.
If all you want from the flash outside is a bit of fill-in, then direct flash from an on-camera gun or the pop-up is very effective and obviously very easy. If you keep the amount of fill low, so that the flash is less dominant and intrusive, then that is actually very effective.
For any meaningful improvement on that with flash outside, what you need is a (much) bigger light source, that is not dependent on the surroundings. The Fong is bigger (so is a plastic milk bottle
) but the one that I like and use is a Lumiquest. They make several and are about as big as you can sensibly use directly on-camera, and they don't waste light. My favourite is the Lumiquest Quik Bounce because it does so many things well, but there's plenty of choice. I really like the fact that they fold up flat too. They're about £20-40
http://www.lumiquest.com/products.htm
I think I've mentioned this before, but it may be relevant to the OP's child photography this summer. I have a shoot-through brolly fixed to an on-camera bracket - that's big (and looks ridiculous) but works fantastically well - a portrable studio almost. I can chase kids around the garden, with mum in tow holding a reflector, and the E-TTL just sorts out the exposure which of course varies so quickly as the distance changes all the time. This is the Manfrotto flash bracket
http://www.warehouseexpress.com/buy-manfrotto-mn233b-camera-flash-bracket/p11017
