Correct me if I'm wrong, but it appears that you are not lighting your background. The catch light in the eyes suggests that you are only using 1 light and that it is way too close to the camera.
I would suggest you study lighting setups and try something different.
yeah, I'm not all that happy with my lighting setup and trying to figure out what I can do different. I have a softbox and umbrella at each corner. I am currently in my parent's basement until me and my fiance find a place. I only have about 10-12ft wide to work with because of a post in the basement. And probably 16-18ft in lenght to work with. So.....not the greatest conditions but am trying to make it work. I did borrow a third light one day from a friend to try and brighten up my white backdrop. To get the light behind the subject it was like a spotlight in the middle of the backdrop.....then when I moved it back I obviously blew out the subject. Don't have enough room on both sides to have a light on both sides. What can I do???? With seniors I could hide the third light behind them but with kids that is obviously not gonna happen. Any suggestions if any of what I just said makes any sense???
If you want a white background, you need a background light.
You can have the softbox be your key, a reflector or piece of foamcore be your fill, and the umbrella light be your background light 2 stops brighter than the key.
I would suggest using one light on the ceiling pointed toward the background so it illuminates the background. If you can, let it spill a little light on the back of the subjects head. To make the background a clean white, if your camera is set for f5.6 for the subject, light the background for f11.
With 18 feet of usable length, try to keep your subject at least 5 feet from the background - 7 or 8 would be better.
Then put the softbox to the side of the subject, but aim it in front of the face, not at the face, so it washes across. Put a reflector on the opposite side to kick light back on the other side for a fill.
That should give you nice dimensional lighting with 2 lights.
If you get a 3rd, use it as a hairlight and dedicate the background light to that use.
I wouldn't worry about a fill light so long as the reflector is kept close. White foam core or a piece of pink foam insulation with a sliver finish works great and is very inexpensive. There are a lot of times that a softbox and a reflector is all you'll need.
The balance comes from experimenting with the distance of the softbox and reflector to the subject. I suggest you start with the softbox about 3-4 feet and the reflector at half that distance.
For kids, get the lights set up first. Then put them on a stool or in a chair and you should get a few nice portraits before they get too antsy.