Thanks for asking this question as I wondering the same thing. Thanks CarissaLyn for your answer.
I tried this just yesterday, I have three boy's (9,8 and 4) that rarely keep still, I set them up a few feet infront of christmas lights on a black sheet background, I used my speedlite with my umbrella (to help soften the lighting, and speedlite set on lowest lighting), and set to the side and a reflector on the opposite side. I had to do this in a dark room. It worked, my lights in the background where blurred and my boy's were sharp and in focus. And as CarissaLyn said if flash is front on it looses the glow in the christmas lights.
I still need alot of practise to get this just right, but I'm posting this to say it can work.
"Life is not measured by the number of breaths we take, but by the moments that take our breath away" - author unknown
My greatest Loves- My husband, our 3 boy's, animals, my canon EOS 650D, my canon EFs 18-55mm, EFs 18-135mm, EF 50mm and EF90-300mm lens, my speedlite 320EX and creating with prisma premier colored pencils.