One thing that I would very seriously consider is looking at the local book stores and grabbing a few books on portrait, kids and lighting.
Each aspect will allow you to fill in the photos with alot more professionalism overall.
Not being either derogatory or otherwise defrocking, but the real focus will be in the quality of the photo int he end.
The various APS cameras are fine and I would also look at KEH.com at the good to Like New selection rating cameras.
figure which brand and stick to it.
But practice heavily on kid photography and the techniques used for kids (they dont sit still), and especially lighting.
if you have any business background, now is the time to use it.
Also figure that if a business, you need to consider contracts, etc.
The camera brand and type really is going to be secondary at this point.
Eventually if you continue with it, the move to larger and more expansive cameras will come to play.
Also be ready to have solid backup capabilities. (Exterior hard-drive, hard copies, etc.)
I already have a ton of practice with kids. I have 4 children of my own, and I regularly use them to get some practice on my poses, angles, etc. I'm all about lighting as it is. I've had physical cameras before, I've just never shelled out the money for a super expensive professional cameras. Just from experience I feel like I have a good handle on different poses, props, set ups, etc. Pretty good at figuring out the lighting, but I'm moving into a new ballpark with purchasing different flashes and lighting. I've looked a few and cant decide what would be best to purchase. Thanks for the input! I plan on definitely exploring and expanding my education on photography
Now with respect to you, keep in mind a couple of things.
With kids, using your own, is all fine and good for the composure side.
But remember that your dealing with someone else's kids who probably dont want to be there and dont know you. (trust me, I have been down that road a few times.)
For equipment that you originally asked:
Getting into decent portrait cameras now is not super expensive and the somewhat older pro end stuff is selling pretty cheap now.
this is really a good time to explore those arenas.
The reason is because the pro end stuff from ten or twelve years ago (2006-2013) were pretty high quality and still are.
Just super cheap now.
getting into Med. Format portrait is one place to explore and not overtly expensive either. Full Frame (5D, 1Ds MkII or such) or any of the Nikon stuff will work just fine.
The real crux is now getting the environment set properly.
Which begs the question... where?
This will heavily impact equipment choice later. Will you go to events or do they come to you?
Get to understand the DoF ratings, and the diff. FD numbers.
A 100mm lens for a DSLR is fine, but don't go the $50 rout unless you know exactly what your buying. You will have to spend some money on the glass.
Most lenses between the 60-135 mm range for typical DSLRs is where you want to focus and look to primes, because the image quality and such things like Chromatic Aberration (CA) and the like is greatly affected by the type of lens.
It sounds like you have the motivation. Now comes learning the specific tools.
Its sorta like using a claw hammer to build a house frame over a pneumatic nail gun. Yes you can do it, but why?
Most importantly... KEEP ASKING!!!
The people here have about a gazillion years of combined experience and can guide you well....