So I have been booked to-do a family photo with a mix of 10 total adults and children. I have a white umbrella, stand and external speed light and a wireless trigger system. A 70 to 200 mm and 35 mm lens. Im thinking out side will be ideal. Any and all suggestions are greatly appreciated.
I've done this. Even with a 7-foot diameter umbrella and a 150ws light, there was significant fall-off over the width of the group. Everyone was totally crammed together as tightly as we could get. Some (such as myself for one) were just heads floating on someone's shoulder. I had a speedlight opposite the umbrella for fill, but pretty far back due to the width of the lens (18mm). Camera was about 20 feet from the group.
Yes, you'll need to be outside where you can get back a ways.
Watch your background.
You might consider posing everyone facing the setting sun for key light, and using the umbrella for fill.
Good luck!