I am not a pro but I have my subjects at least 4 ft away from the backdrop. I also shoot with two lights at 45 degree angles from the subject (with kids I am just looking for a ton of light because they move so quick!) I think the cross light helps with any shadows. Hopefully one of the more skilled ladies can give you some more info. Here is a link to some pictures I took just the other day with this method.
http://flickr.com/photos/jenniferdel...7614021501524/
HTH 
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My Camera - Canon 50D - soon to own 5D Mark III 
My Lenses - 50mm 1.2, 70-200mm f4, 24-70mm 2.8, 100mm 2.8
My Software - Photoshop CS5, Lightroom 3
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