Christy! I am so glad that you have started this.
You have set for yourself a mighty task I must say. Drawing a model that sits still is hard enough, but children will present a consistently moving target. I think that ultimately for commissions you will have to work from a combination of life and photos - getting enough visual information to produce a finished piece from life alone is gonna be tough with the younger subjects.
And what a wonderful learning tool for you! It can only improve your work and hopefully your enjoyment of it all.
I like the first portrait drawing you have posted below. I agree that the others lack the intermediate level of finish that you were able to get in the first portrait - there is a lot to capture in a full figure and a whole other challenge of what to represent to get that level of finish.
How would you take something like one of those sketches and produce from it, and your memory, a high finish portrait?
One option might be to have an initial session where you do sketches like that, and also do a color sketch to capture skin tones and temperature from a live setting. Take some reference pics during that session as well. Take your materials off and work up an intermediate stage portrait. Get your subject back in and compare everything you have done - get refining notes either on paper or in your head. If you can, have them sit and work some more from life - even with them moving you will be able to observe so much that you don't get from a static photo.
Take it all back and work the intermediate into a final finished portrait.
Dunno, it is a challenge but a worthy one I think! Congrats on starting this!