I’m not sure how to evenly light a small group and... (
The inverse square law will illustrate why the light falloff is so great in your image. It looks like you are in tight quarters, and your subject on the left side is pretty close to the light.
There are several approaches to fixing this.
1. If you have the room, is to move the key light further away from the group. This will have two effects - a lower rate of decay on the light, but your shadows will become harsher and deeper. You will need to use either a large softbox or a much bigger umbrella. I would put a kicker as a hair light in the back to create some interest and separate the child on the right from the darker background, put a small, wide angle light on the background, and possibly a reflector on the right to soften the shadows.
2. You could use a large reflector - I use beadboard insulation from Home Depot, and use the monolight on that - the large surface will create the smoothest, softest shadows, and still provide some directionality.
I would definitely have each of your subjects dressed with clothing that has similar light reflection - putting the guy in the white shirt closest to the light is creating problems for you.
I like the Rembrandt lighting - it works nicely here. But you could use some fill and accent lighting.
Here is a link to making a reflector like the one I mentioned above:https://www.youtube.com/watch?v=BJO9IZPQnKM
You can get some tips here, if you have the patience to endure this guy's presentation skills:https://www.youtube.com/watch?v=TAcY5ob2UnE
Another approach is to get a large 5 in 1 collapsible umbrella, like this one:https://www.youtube.com/watch?v=ov-CyVRIVCA
Because you are dealing with a group, bigger is better - which is why I like the beadboard approach - even indoors. The light is nice and even and very controllable.
If you decide to stay with an umbrella, I'd suggest you get a much bigger one - maybe like this:https://www.amazon.com/dp/B078XT7VSM/ref=sspa_dk_detail_0?psc=1&pd_rd_i=B078XT7VSM&pf_rd_m=ATVPDKIKX0DER&pf_rd_p=21517efd-b385-405b-a405-9a37af61b5b4&pd_rd_wg=VVB9R&pf_rd_r=VG50AHPCWF8J6W0CS4G0&pf_rd_s=desktop-dp-sims&pf_rd_t=40701&pd_rd_w=JcUrN&pf_rd_i=desktop-dp-sims&pd_rd_r=0da5d78d-f175-11e8-bed5-7dcfcd74da26
Hope this helps.