Chiming in pretty late here. Much has been said already.
It depends on what you are really looking for. If you want that clean white background, Derrel describes as 20-TENS

, you really need to get further away with your background and make sure the light of your slaves does not spill back to your kids - anything will do from cardboard to light modifiers.
The best thing would be thick styrofoam boards that stand on the floor. I only know the metric dimensions in cm - you can get them very cheap at 100cm x 50cm x 10cm. Put two of them (on top of each other) on either side of the kids.
The foam boards also act as a reflector and bounce light from the background and from your softbox to your subjects.
Place your softbox as close to your kids as possible and don´t worry if it enters your frame - you can easily remove that later (even in lightroom - no need for photoshop. Lightroom even has the advantage that you can copy and paste your settings including the removal of the softbox from one image to another) as long as the background is clean white.
What you need to do is expose for the background first, because you can´t adjust the settings on your slaves. Then you dial in the softbox-light to fit the background.
Here is a draft:
If you don´t want to create a clean white background, you´d rather use a wall instead of background paper,
so you can put your kids right at the wall to get a nicer look and don´t get any wrinkles in the paper.
In regard to distance background - kid - camera. A lot has been said. The bigger the distance, the better it is.
I have shot tons of images on white like the ones below - you can even shoot white clothing on white. I love the clean look of these images. Some think they are boring, but I rather find it interesting to give the focus to the subjects.
I especially love if these images are placed on white background on webpages, etc. It doesn´t look as good on the offwhite background here.