group xmas portrait help!

[quixilver]

Hi guys,
Friday I'm going to shoot a family of 10 for their xmas photos.
I would love to have some opinions about how to light them.
I never photographed such a big group and also I never posed one as well.
This is the diagram of the lighting setup I was thinking to do

But I'm a little concerned and would love your advices.
What I did here is a softbox up high at 45 degress reading f8 as key light, then a shoot trough umbrella as fill light reading 5.6 and a separation light reading 5.6 as well.
Do you guys think it might work?
Is there a better 3 light setup for this kind of shoot?
I also just tried this one at home with my wife and I really didn't like it....it came out flat, but that could also be my very low ceiling and perhaps also all my white walls bouncing all over, I don't know, all I know is that I'm getting a panic attack ahahahhaha!!!!
Please write me and let me know what you think should be best
Thank you