Christy, I'm really happy to hear that you're going to do this - I think you will be very good at it since you seem to have an instinctive feel for the emotional qualities that are distinctive in children and seem to fossilize as we age.
I teach a course in this and so I have a lot of advice which seems to me is scattered around this Forum hither and yon. (There are many other good artists on this site who also paint children from life and I steal ideas from them whenever I can.
) The main idea is to do a LOT of work from life before you start charging for them and get used to the idea that a lot of studies will go in the rubbish. You have to get used to the idea that you are working on moving targets. Personally, I alternate reading "fine art" books with books on animation and cartooning since I think that there are a lot of concepts which cross-pollinate.
As far as the business side is concerned, I now have different catagories of portraits on my website and I am offering a discount to clients who come for life sittings in my studio. I've done two of these in the past few months (one is still ongoing, he comes by every five weeks or so) and it is working out very well for me. As soon as I figure out how to do this while travelling on the road (multiple sittings are the issue) I'll offer this as well.
Meanwhile, I'd advise anybody that the first step should be to get yourself in front of lots of children and draw until you are rock-solid in your knowledge about children's head porportions.