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I agree with jefferies1 for the most part. Keep it simple. Remember, that's what the "mall Santas" do. One single strobe, mounted high and centered, will cover the kids on either knee. I think a softbox would be better than an umbrella, but a Westcott Halo or Lastolite 8 in 1 would also give you a similar look. You probably don't have those where you are, but the spill from these is less than a regular reflective or shoot through umbrella but more than a flanged softbox. Steady's also right; check out the location a day or so before, to see what the ambient gives you, or the problems it may cause.