If you see yourself doing more family shootings, I agree with the others here, an off camera flash is good to give you more flexibility with your lighting.
Also, as mentioned, the d7000 would be a step up for you in terms of more/easier control of the camera than your d3100. I understand the video capabilities of the d7000 are better than the d90.
Looks like the flash will be the next step. Would love to have an off camera flash in the sense of a remotely controlled speedflash/umbrella set-up, but with the D3100, that's definitely going to involve a couple of wireless transceivers plus the flash. It really just depends on how many shoots I get in the cold months, etc. May be that I continue doing what I do, then when the spring hits, and business ramps up, I go from there, but *hopeful* that I have the next step in place before the spring weather returns!
Never know, although it's a stretch, perhaps I'll find a way to fund the D7000 and an SB-700 and knock it all out at once!
Then I could work on finding a D90 as a back-up and try to get my son interested in the D3100.
I would also suggest investing in a good light meter. Outdoor lighting can be tricky, and when you're working with additional portable lighting, it gets even trickier. A good light meter can help smooth out the rough spots and save you a lot of trial and error in front of clients.
That is the truth. A nice shady area helps, but when you've got patches of sunlight trickling in, it definitely makes for a challenge. For now, I get really close with being picky about where I set up, matrix metering and some spot adjustments in LR. I learned rather quickly that you'd rather be a little underexposed than a little over. Most of the time, I'll have my assistant (son or wife, LOL) hold up a flexible gray card in a couple of areas (sun/shade), take quick readings, then find the right split from there. The trick so far is in having a few choice places that I "suggest" for the shoot, depending on whether they want urban, industrial, brick, natural... then knowing what times of day make for the best shots. I've used them enough, and have taken enough practice shots with the fam, that I'm comfortable. If a client suggests a new spot, I'll usually try to arrive early and gray card it.
I guess a meter would help save me some time and trouble, though!